TMD vertical Van der Waals heterojunction and preparation and application thereof

Abstract

The invention belongs to the field of two-dimensional materials, and particularly relates to a preparation method of a TMD vertical Van der Waals heterojunction, and the method comprises the steps: carrying out the physical vapor deposition of MX2 on the surface of two-dimensional NY2 under the assistance of an ammonium salt, and obtaining an MX2 / NY2 vertical Van der Waals heterojunction; the MX2 and the NY2 are different types of transition metal chalcogenide compounds. The invention also comprises the heterojunction prepared by the preparation method and the TMD application of the heterojunction. The TMD vertical Van der Waals heterojunction is successfully synthesized for the first time.

Description

Technical Field

[0001] The invention belongs to the field of van der Waals heterojunction preparation, and in particular relates to the field of transition metal chalcogenide compound vertical van der Waals heterojunction preparation. Technical Background

[0002] Two-dimensional (2D) transition metal dichalcogenide (TMD) vertical heterostructures have attracted extensive attention in fundamental research and applications in electronics, optoelectronics, etc. due to their unrestricted lattice matching. However, current research on 2D TMD vertical heterostructures is mainly limited to mechanical exfoliation and restacking methods, which limits their yield and scalability. To advance the field and achieve scalable device integration, significant efforts have been made to develop strategies for the scalable synthesis of 2D TMD vertical heterostructures. Chemical vapor deposition (CVD) methods provide a more scalable and controllable method to produce high-quality, large-area 2D TMD vertical heterostructures.

[0003] However, the CVD synthesis method usually involves a complex growth process, including multiple factors such as gas flow, growth temperature, pressure and precursors. All of these factors will affect the final structure, and most heterostructures synthesized by the CVD process suffer from problems such as thermal degradation, uncontrolled growth direction and limited material combinations, which makes the synthesis of large-scale, high-quality vertical heterostructures still a major challenge. Achieving the selective growth of high-quality lateral and vertical heterojunctions in the CVD process remains a difficult problem because it is difficult to control the nucleation sites, which can lead to uneven growth, defects and interface unevenness. Factors such as different growth rates, interface energy mismatch, changes in precursor reactivity and the complexity of process parameters also exacerbate this challenge. Summary of the invention

[0004] In view of the problem that it is difficult to form a TMD vertical van der Waals heterojunction in the prior art, the first object of the present invention is to provide a universal method for preparing a TMD vertical van der Waals heterojunction, aiming to prepare a TMD vertical van der Waals heterojunction.

[0005] The second purpose of the present invention is to provide a TMD vertical van der Waals heterojunction prepared by the preparation method and its application in the preparation of electrical devices.

[0006] The third object of the present invention is to provide an electrical device comprising the TMD vertical van der Waals heterojunction.

[0007] The vertical van der Waals heterojunction of TMD semiconductor materials is difficult to grow by chemical vapor deposition, mainly because of the unique physical and chemical properties of TMD semiconductor materials, the complexity of growth conditions, the challenges of interface quality and the limitations of chemical vapor deposition preparation process, which make it difficult to meet the needs of growing different materials at the same time and form a stable, high-quality heterojunction at the interface. Due to the difference in MX2 (M = W or Mo, X = S or Se) mismatch, the heterojunctions grown by traditional growth methods are mostly lateral heterojunctions. In view of the problem that TMD semiconductor materials are difficult to prepare, the present invention has been studied in depth and provides the following solutions:

[0008] The preparation method of the TMD vertical van der Waals heterojunction is to physically vapor deposit MX2 on the surface of the two-dimensional NY2 with the assistance of ammonium salt to prepare the MX2 / NY2 vertical van der Waals heterojunction;

[0009] The MX2 and NY2 are different types of TMD.

[0010] In order to solve the problem of difficulty in forming a vertical van der Waals heterojunction due to the physicochemical properties of TMD, the present invention innovatively physically vapor deposits MX2 on NY2 with the assistance of ammonium salt, which can unexpectedly induce MX2 to form a vertical van der Waals heterojunction on NY2.

[0011] In the present invention, the MX2 and NY2 can be any different types of TMDs that can theoretically form a heterojunction. For example, the MX2 and NY2 are individually at least one of WS2, WSe2, MoS2, and MoSe2; and the two are selected from different materials.

[0012] In the present invention, the two-dimensional NY2 is a non-overhanging surface, which can be obtained based on conventional CVD or PVD methods.

[0013] The present invention innovatively discovered that a 2D TMD vertical van der Waals heterojunction can be obtained for the first time by using a simple physical vapor deposition (PVD) method with the assistance of ammonium salts. The present invention's research shows that a series of active intermediates can be generated by the introduction of ammonium salts. These active intermediates have lower adsorption energy on the surface of TMD materials and will preferentially nucleate on the surface of TMD substrates, thereby promoting the growth of vertical TMD vertical van der Waals heterojunctions.

[0014] In the present invention, the ammonium salt comprises at least one of an organic ammonium salt and an inorganic ammonium salt; further, it is ammonium chloride.

[0015] In the present invention, the weight ratio of the ammonium salt to MX2 is 0.05 to 1, further 0.1 to 0.5:1, and further 0.1 to 0.2:1. Studies have shown that the weight ratio is helpful to further improve the selectivity of the vertical van der Waals heterojunction.

[0016] In the present invention, MX2 and ammonium salt can be volatilized and physically vapor deposited on the surface of two-dimensional NY2 under the carrier gas to obtain the MX2 / NY2 vertical van der Waals heterojunction.

[0017] In the present invention, the volatilization temperature of MX2 is 1000-1200°C;

[0018] In the present invention, the volatilization temperature of the ammonium salt is 80 to 180°C, and may be 110 to 150°C.

[0019] In the present invention, the carrier gas is a protective atmosphere, preferably Ar;

[0020] In the present invention, the flow rate of the carrier gas is 50 to 200 sccm, and can further be 60 to 100 sccm.

[0021] The research of the present invention also shows that under the innovation of the ammonium salt-assisted PVD, further optimization of the deposition temperature and time is helpful to obtain a vertical heterojunction while also helping to regulate the thickness and morphology of MX2, thereby helping to obtain atomically thin MX2.

[0022] In the present invention, the deposition temperature in the physical vapor deposition stage is 650-850°C, preferably 700-800°C;

[0023] In the present invention, the deposition time in the physical vapor deposition stage is 5 to 25 minutes, further 8 to 16 minutes.

[0024] The present invention also provides the TMD vertical van der Waals heterojunction.

[0025] The present invention also provides the application of the TMD vertical van der Waals heterojunction, which is used to prepare optical and / or electrical devices.

[0026] In the present invention, the TMD vertical van der Waals heterojunction described in the present invention can be used to manufacture desired devices based on known processes and means, for example, optical devices, electrical devices, etc.

[0027] For example, the present invention provides an optional electrical device, the preparation steps of which are:

[0028] The sample was marked by electron beam exposure on the TMD vertical van der Waals heterojunction surface, and then metal was deposited on its surface to obtain a field effect transistor;

[0029] In the present invention, metal is deposited on the surface of the TMD vertical van der Waals heterojunction by a vacuum coating machine;

[0030] In the present invention, the metals are In and Au.

[0031] The present invention also provides an optoelectronic device, comprising the TMD vertical van der Waals heterojunction, or being made by the TMD vertical van der Waals heterojunction.

[0032] Beneficial Effects

[0033] 1. The present invention shows that vertical van der Waals heterojunction can be grown on the surface of TMD two-dimensional materials based on the PVD method with the assistance of ammonium salts.

[0034] 2. The present invention uses ammonium salt-assisted atmospheric pressure chemical vapor deposition at an optimal growth temperature, with the coordination of carrier gas flow and the uniform plane size and surface thickness of the two-dimensional material of the non-dangling bond substrate (NY2), to produce a vertical van der Waals heterojunction with uniform morphology, controllable thickness and good crystallinity.

[0035] 3. The present invention prepares a series of TMD vertical van der Waals heterojunctions with good morphology, regular hexagons or triangles, good crystallinity and high quality. For example, the method can be used to prepare WSe2 / WS2 PN junction field effect transistors for photodetectors. The present invention prepares ultra-thin, especially atomic-thick TMD growth, which is crucial for the basic research and potential technological applications of this type of new materials with 2D restrictions. The preparation process of the present invention does not require complicated operating steps and the use of expensive raw materials, the equipment is simple, and the operation is simple and easy with good reproducibility.

[0036] 4. The present invention obtains different types of vertical van der Waals heterojunctions by extending on a TMD substrate without dangling bonds through a simple atmospheric pressure chemical vapor deposition method. The heterojunctions are single crystals with high quality, controllable thickness and good reproducibility. The preparation method is simple and feasible, and provides a reference for the preparation of van der Waals heterojunctions of other two-dimensional semiconductor materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of a normal pressure chemical vapor deposition apparatus for preparing a TMD substrate in Example 1;

[0038] Figure 2 Optical image of WS2 on SiO2 / Si substrate prepared in Example 1;

[0039] Figure 3 Atomic force image of WS2 on SiO2 / Si substrate prepared in Example 1;

[0040] Figure 4 Optical image of WSe2 on SiO2 / Si substrate prepared in Example 1;

[0041] Figure 5 Schematic diagram of a van der Waals heterojunction device prepared on a SiO2 / Si substrate in Example 2;

[0042] Figure 6 This is an optical image of a MoS2 / WS2 vertical heterojunction prepared on a SiO2 / Si substrate in Example 2;

[0043] Figure 7 Atomic force diagram of the vertical heterojunction of MoS2 / WS2 prepared on SiO2 / Si substrate in Example 2;

[0044] Figure 8 This is an optical image of a MoS2 / WS2 hybrid heterojunction prepared on a SiO2 / Si substrate in Example 3;

[0045] Fig. 9 Atomic force diagram of the MoS2 / WS2 hybrid heterojunction prepared on a SiO2 / Si substrate in Example 3;

[0046] Fig.10 This is an optical image of a MoS2 / WS2 lateral heterojunction prepared on a SiO2 / Si substrate in Comparative Example 1;

[0047] Fig.11 The atomic force diagram of the MoS2 / WS2 lateral heterojunction prepared on the SiO2 / Si substrate in Comparative Example 1;

[0048] Fig.12 This is an optical image of a van der Waals vertical heterojunction prepared on a SiO2 / Si substrate by Group A of Example 4;

[0049] Fig.13 This is an optical image of a van der Waals vertical heterojunction prepared on a SiO2 / Si substrate by Group B of Example 4;

[0050] Fig.14 Optical images of van der Waals vertical heterojunctions at different growth temperatures (low temperature) prepared on SiO2 / Si substrates by Group A of Example 5;

[0051] Fig.15 Optical images of van der Waals vertical heterojunctions at different growth temperatures (high temperature) prepared on SiO2 / Si substrates by Group B of Example 5;

[0052] Fig.16 Optical images of different types of vertical van der Waals heterojunctions prepared on SiO2 / Si substrates in Example 6;

[0053] Fig.17 The Raman spectra and imaging corresponding to different heterojunctions in Example 6;

[0054] Fig.18 The STEM images corresponding to different heterojunctions in Example 6;

[0055] Fig.19Optical photographs and schematic diagrams of the WSe2 / WS2 vertical van der Waals heterojunction photodetector of Example 7;

[0056] Fig. 20 This is the output curve of the WSe2 / WS2 vertical van der Waals heterojunction field effect transistor of Example 7;

[0057] Fig.21 This is the transfer curve of the WSe2 / WS2 vertical van der Waals heterojunction field effect transistor of Example 7;

[0058] Fig. 22 Schematic diagram of the photoelectric gain of the WSe2 / WS2 vertical van der Waals heterojunction photodetector of Example 7.

[0059] Fig.23 This is a response time diagram of the WSe2 / WS2 vertical van der Waals heterojunction photodetector of Example 7.

[0060] Fig.24 Graph showing the detectivity and responsivity of the WSe2 / WS2 vertical van der Waals heterojunction photodetector of Example 7;

[0061] Fig.25 The external quantum efficiency of the WSe2 / WS2 vertical van der Waals heterojunction photodetector of Example 7; Specific implementation plan

[0062] The present invention is further described below through implementation cases, but the content of the present invention is not limited to the following content.

[0063] 1. Preparation of TMD substrate on SiO2 / Si:

[0064] Example 1

[0065] The experimental device for preparing TMDs substrate is shown in the figure. Figure 1 shown. Figure 1The above figure is a diagram of the preparation device of the WS2 nanosheet substrate. The WS2 nanosheet substrate (also referred to as the WS2 two-dimensional material substrate, or WS2 substrate in the present invention) can be prepared by the existing method. For example, the steps may be: place a porcelain boat containing WS2 in the constant temperature zone 1 (central temperature zone, upstream) of the tube furnace, and place a piece of 285nm SiO2 / Si as a growth substrate on another porcelain boat and place it downstream of the tube furnace to obtain an appropriate crystal growth temperature. Before heating, use a large flow of argon to exhaust the air in the quartz tube. Then heat the constant temperature zone 1 to 1180°C, and the argon flow rate is 80sccm. During the heating process, a reverse airflow (blowing from the substrate to the source) is used. After the heating is completed, it is switched to a forward airflow (from the source to the substrate). After growing for three minutes, a single crystal WS2 nanosheet will be generated on the SiO2 / Si substrate. After the growth is completed, the WS2 substrate is quickly pulled out and the airflow direction is changed to the reverse direction. The optical photographs and atomic force microscopes of the prepared WS2 nanosheets are shown in Figure 2. Figure 2 and Figure 3 shown.

[0066] Figure 2 This is an optical schematic diagram of the WS2 nanosheets prepared in this case. The WS2 nanosheets obtained under this condition have good crystallinity, atomic-level thickness, and a lateral size of 130μm. Figure 2 The scale bar in the figure is 20 μm. Figure 3 Atomic force diagram of the WS2 nanosheet prepared for this case, showing that the thickness of the prepared WS2 substrate is 0.8 nm, corresponding to a single layer of WS2 nanosheet.

[0067] A similar method as described above was used, but the source was replaced with WSe2 to prepare a WSe2 nanosheet substrate, wherein the PVD deposition temperature of WSe2 was 1160°C.

[0068] The optical image of the prepared WSe2 nanosheets is shown in Figure 4 The WSe2 nanosheets obtained under this condition have good crystallinity, atomic-level thickness, and a lateral size of 160 μm. Figure 4 The scale bar in the figure is 20 μm.

[0069] 2. Preparation of van der Waals heterojunction

[0070] Example 2

[0071] Using three temperature zone tube furnace ( Figure 5 ), wherein the upstream Zone 1 is provided with a magnetic boat A loaded with ammonium chloride; the middle Zone 2 is provided with a magnetic boat B loaded with MoS2 (TMDs raw material); the downstream Zone 3 is provided with a magnetic boat C, on which the WS2 nanosheet substrate prepared in Example 1 is provided (WS2 nanosheet facing upward); wherein the amount of MoS2 raw material used is 0.5 g, and the weight ratio of ammonium chloride to MoS2 is 0.1:1;

[0072] The tube furnace was heated to make the temperature of Zone 1 (the volatilization temperature of ammonium chloride) 120°C, the temperature of Zone 2 (the volatilization temperature of MoS2) 1190°C, and the temperature of Zone 3 (the growth temperature) 700°C. The deposition was carried out in a carrier gas (Ar), wherein the flow rate of the carrier gas was 80 sccm and the deposition time was 10 min. A MoS2 / WS2 vertical heterojunction was obtained.

[0073] Figure 6 This is an optical image of the MoS2 / WS2 vertical van der Waals heterojunction prepared on a SiO2 / Si substrate. The MoS2 / WS2 vertical van der Waals heterojunction obtained under this condition has good crystallinity, thin thickness, and a size of 180μm. Figure 6 The scale bar in the figure is 20 μm. Figure 7 The corresponding atomic force image shows a thickness of 1.5 nm, corresponding to the monolayer-monolayer MoS2 / WS2 vertical van der Waals heterojunction.

[0074] Example 3

[0075] Compared with Example 2, the only difference is that the amount of ammonium chloride is changed, specifically: the weight ratio of ammonium chloride to MoS2 is 0.2:1.

[0076] Figure 8 This is an optical image of the MoS2 / WS2 mixed vertical van der Waals heterojunction prepared on a SiO2 / Si substrate. The MoS2 / WS2 vertical van der Waals heterojunction obtained under this condition has good crystallinity, thin thickness, and a size of 120μm. Figure 8 The scale bar in the figure is 10 μm. Fig. 9 This is the atomic force image corresponding to the prepared sample. The prepared van der Waals heterojunction has obvious steps, corresponding to the single-layer-monolayer MoS2 / WS2 mixed van der Waals heterojunction.

[0077] Comparative Example 1

[0078] Compared with Example 2, the only difference is that no ammonium chloride is added. Other operations and parameters are the same as in Example 1.

[0079] Fig.10 This is an optical image of the MoS2 / WS2 lateral heterojunction prepared on a SiO2 / Si substrate. The MoS2 / WS2 lateral heterojunction obtained under this condition has good crystallinity, thin thickness, and a size of 120μm. Fig.10 The scale bar in the figure is 10 μm. Fig.11 is the corresponding atomic force image. The prepared van der Waals heterojunction has no obvious steps, corresponding to the single-layer-single-layer MoS2 / WS2 lateral heterojunction.

[0080] Example 4

[0081] Compared with Example 2, the only difference is that the key deposition conditions, such as deposition temperature and time, are changed. The experimental groups are:

[0082] Group A: The heating temperature of ammonium chloride was 140°C and the deposition time was 8 min;

[0083] Group B: The heating temperature of ammonium chloride was 120°C and the deposition time was 15 min;

[0084] As the growth time increases, the coverage area gradually increases, such as Fig.12 , as shown in 13, the scale bar in the figure is 20μm.

[0085] Example 5

[0086] Compared with Example 2, the only difference is that the key deposition conditions, such as deposition temperature and time, are changed. The experimental groups are:

[0087] Group A: MoS2 heating temperature is 1100℃, deposition temperature is 720℃, and deposition time is 10min;

[0088] Group B: MoS2 heating temperature is 1200℃, deposition temperature is 730℃, and deposition time is 10min;

[0089] As the growth temperature increases, the MoS2 nanosheets become larger, such as Fig.14 , the scale bar in Figure 15 is 10 μm.

[0090] Example 6

[0091] Compared with Example 2, the only difference is that the TMD and the substrate are changed. The experimental groups are:

[0092] Group A: TMD is MoS2; the substrate is deposited with WSe2, and finally a MoS2 / WSe2 vertical heterojunction is obtained.

[0093] Group B: TMD is WSe2; the substrate is deposited WS2, and finally a WSe2 / WS2 vertical heterojunction is obtained.

[0094] Group C: TMD is MoSe2; the substrate is deposited with WSe2, and finally a MoSe2 / WSe2 vertical heterojunction is obtained.

[0095] Group D: TMD is MoSe2; the substrate is deposited WS2, and finally a MoSe2 / WS2 vertical heterojunction is obtained.

[0096] Different TMD powders were placed in the midstream temperature zone as growth raw materials, with a flow rate of 80 sccm and a deposition time of 10 min. SiO2 / Si substrates with WS2 or WSe2 nanosheets were placed in the downstream temperature zone. Fig.16 Optical images of different types of vertical van der Waals heterojunctions prepared on SiO2 / Si substrates. The vertical van der Waals heterojunctions obtained under this condition have good crystallinity and thin thickness. Fig.17 For the corresponding Raman spectrum and imaging, the corresponding vertical van der Waals heterojunction. We also used STEM to analyze the prepared vertical van der Waals heterostructure structure ( Fig.18 ). In particular, the STEM image at the interface proves that the prepared van der Waals heterojunction has an atomically clean interface.

[0097] 3. Preparation of field effect transistors and their application in two-dimensional materials

[0098] Example 7

[0099] The preparation method of WSe2 / WS2 photodetector is to deposit metal In (10nm) / Au (50nm) on the WSe2 / WS2 vertical van der Waals heterojunction (material prepared in Example 6) prepared by PVD method by electron beam exposure to obtain WSe2 / WS2 photodetector. The optical picture and schematic diagram of WSe2 / WS2 photodetector are shown in Fig.19 shown.

[0100] Fig. 20 Output curve of WSe2 / WS2 PN junction field effect transistor; it shows good rectification effect, indicating the formation of vertical WSe2 / WS2 PN junction.

[0101] Fig.21 The transfer characteristic curves of the WSe2 / WS2 PN junction field effect transistor are shown in FIG. 1 , and FIG. 2 are transfer characteristic curves of the WSe2 / WS2 PN junction field effect transistor; both prove the WSe2 / WS2 van der Waals heterojunction prepared by the present invention.

[0102] Fig. 22 This is the optical gain curve of WSe2 / WS2 photodetector.

[0103] Fig.23 The photoelectric response switching diagram and photoelectric response time diagram of the WSe2 / WS2 photodetector; the photoelectric response time is 146μs, showing an excellent response speed.

[0104] Fig.24 Characterization of the responsivity and detectivity of WSe2 / WS2 photodetectors.

[0105] Fig.25 Characterization of the external quantum efficiency of WSe2 / WS2 photodetector.

Claims

1. A method for preparing a TMD vertical van der Waals heterojunction, characterized in that: MX2 was physically vapor deposited on the surface of two-dimensional NY2 with the assistance of ammonium salt to prepare MX2 / NY2 vertical van der Waals heterojunction; The MX2 and NY2 are different types of TMD.

2. The method for preparing a TMD vertical van der Waals heterojunction according to claim 1, characterized in that: The MX2 and NY2 are independently at least one of WS2, WSe2, MoS2, and MoSe2; and the two are selected from different materials.

3. The method for preparing a TMD vertical van der Waals heterojunction according to claim 1, characterized in that: The ammonium salt comprises at least one of inorganic ammonium salts; further ammonium chloride.

4. The method for preparing a TMD vertical van der Waals heterojunction according to claim 1, characterized in that: The weight ratio of the ammonium salt to MX2 is 0.05-1; further 0.1-0.5:

1.

5. The method for preparing a TMD vertical van der Waals heterojunction according to any one of claims 1 to 4, characterized in that: MX2 and ammonium salt are volatilized and physically vapor deposited on the surface of two-dimensional NY2 under the influence of carrier gas to obtain the MX2 / NY2 vertical van der Waals heterojunction.

6. The method for preparing a TMD vertical van der Waals heterojunction according to claim 5, characterized in that: The volatilization temperature of MX2 is 1000-1200°C; Preferably, the volatilization temperature of the ammonium salt is 80-180°C.

7. The method for preparing a TMD vertical van der Waals heterojunction according to claim 6, characterized in that: The carrier gas is a protective atmosphere, preferably Ar; Preferably, the flow rate of the carrier gas is 50 to 200 sccm; Preferably, the deposition temperature in the physical vapor deposition stage is 650-850°C, preferably 700-800°C; Preferably, the deposition time in the physical vapor deposition stage is 5 to 25 minutes, further 8 to 16 minutes.

8. A TMD vertical van der Waals heterojunction as claimed in any one of claims 1 to 7.

9. An application of the TMD vertical van der Waals heterojunction according to any one of claims 1 to 7, characterized in that: Using it to prepare optical and / or electrical devices; Preferably, the preparation step is: The sample was marked by electron beam exposure on the TMD vertical van der Waals heterojunction surface, and then metal was deposited on its surface to obtain a field effect transistor; Preferably, depositing metal on the surface of the TMD vertical van der Waals heterojunction by a vacuum coating machine; Preferably, the metals are In and Au.

10. An optoelectronic device, characterized in that: The invention comprises the TMD vertical van der Waals heterojunction according to any one of claims 1 to 7, or is prepared by the TMD vertical van der Waals heterojunction.

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