Method for improving photoluminescence intensity of single-layer tungsten diselenide WSe2 single crystal

Abstract

The invention discloses a method for improving the photoluminescence intensity of a single-layer tungsten diselenide WSe2 single crystal, and belongs to the technical field of two-dimensional material photoelectric devices, the single-layer WSe2 single crystal grown by a CVD (chemical vapor deposition) method is soaked or spin-coated by a methanesulfonic acid aqueous solution with the concentration of 10-40%, and the treated single-layer WSe2 single crystal is subjected to a photoluminescence test to obtain the photoluminescence intensity of the single-layer WSe2 single crystal. The single-layer WSe2 single crystal which is not subjected to acid treatment is weakest in luminescence, the fluorescence intensity is gradually enhanced and the half-peak width is weakened along with the increase of the concentration, 4.55 times of fluorescence enhancement is realized by soaking the single-layer WSe2 single crystal in methanesulfonic acid with the mass fraction of 40%, and the result of a second harmonic test shows that the result is more accurate. The single-layer WSe2 single crystal shows an obvious sextuple structure with the second harmonic intensity related to the polarization angle under the parallel condition, which indicates that the acid treatment under the dosage does not cause damage to the lattice structure. In addition, by building the back gate field effect transistor, it can be seen that the electrical performance after acid treatment is improved.

Description

Technical Field

[0001] This invention belongs to the field of two-dimensional material optoelectronic device technology, specifically relating to a method for modifying and enhancing the optical properties of a single-layer tungsten diselenide (WSe2) single crystal, and particularly to a method for significantly improving the photoluminescence intensity of a single-layer WSe2 single crystal through a controllable concentration of acid treatment. Background Technology

[0002] In recent years, atomically thin two-dimensional transition metal chalcogenides have become important candidate materials for next-generation nano-optoelectronic devices due to their excellent electrical and optical properties. Among them, monolayer WSe2, being a direct bandgap semiconductor at room temperature and possessing unique spin-valley physics, shows great promise for applications in light-emitting diodes, nanolasers, and quantum information devices. The photoluminescence quantum efficiency of the material directly determines the performance limit of such light-emitting devices. However, monolayer WSe2, whether prepared by mechanical exfoliation or chemical vapor deposition, typically exhibits weak intrinsic photoluminescence intensity. This is mainly attributed to the inevitable generation of non-radiative recombination centers such as sulfur vacancies and lattice defects during the growth and transfer process, causing photogenerated electron-hole pairs (excitons) to recombine before being emitted as photons, severely limiting its application in high-efficiency light-emitting devices. To enhance the photoluminescence of monolayer WSe2, various schemes have been proposed in existing technologies. For example, atomic layer deposition can be used to cover passivation layers to shield defects, but this method only improves the external environment and has limited effect on repairing intrinsic defects in the material. Surface passivation using organic molecules is limited by the type of molecule and its adsorption stability, and may introduce impurities. Sulfur-atmosphere annealing can also repair selenium vacancies, but the high-temperature process is incompatible with heat-sensitive flexible substrates. Furthermore, most of these methods are complex and have narrow parameter control ranges, making it difficult to achieve precise, gradient, and repeatable control of the material's optical properties. In summary, existing technologies lack a method that can directly and effectively enhance the photoluminescence intensity of monolayer WSe2 under ambient temperature and pressure through simple and controllable chemical treatment, and can achieve different degrees of enhancement. Therefore, this invention proposes a monolayer WSe2 photoluminescence enhancement technology based on treatment with acid solutions of different concentrations. This technology utilizes methanesulfonic acid solution to controllably treat the material surface within a suitable concentration range. By passivating defect states, it significantly suppresses nonradiative recombination, thereby greatly improving its photoluminescence efficiency and electrical properties. Furthermore, the enhancement effect can be linearly regulated by changing the acid concentration, providing a new approach for the fabrication of customized optical devices. Summary of the Invention

[0003] The purpose of this invention is to solve the above-mentioned problems and provide a method for improving the photoluminescence intensity of monolayer tungsten diselenide WSe2 single crystal.

[0004] 1. A method for improving the photoluminescence intensity of a single-layer tungsten diselenide (WSe2) monocrystalline crystal, comprising the following steps:

[0005] 1) Soak the monolayer WSe2 single crystal in a 10%-40% (w / w) methanesulfonic acid aqueous solution for 10-60 minutes;

[0006] Alternatively, it can be spin-coated onto the surface of a single-layer WSe2 monocrystalline crystal using a spin coater;

[0007] 2) Dry with nitrogen gas;

[0008] 3) Place it on a heating table with a temperature of 90-140℃ and heat for 5-10 minutes;

[0009] The concentration of the methanesulfonic acid aqueous solution is 20%;

[0010] The spin coating is performed at a speed of 500 rpm for a spin coating time of 30 seconds; or at a high speed of 1500 rpm for a spin coating time of 60 seconds.

[0011] The monolayer WSe2 single crystal is prepared by the following method:

[0012] 1) A horizontal muffle furnace was used as the reaction apparatus, and gold foil was selected as the growth substrate. The precursor reaction sources were metal oxide WO3 and chalcogen element nonmetal Se, which were placed in a high-temperature zone of 850-900℃ and a low-temperature zone of 350℃, respectively, to participate in the reaction. During the heating stage, inert gas was introduced at a gas flow rate of 50-200 ml / min, and the temperature was increased at 15-40℃ / min to reach 800-900℃. When the material entered the growth stage, hydrogen and inert gas were introduced. The hydrogen flow rate was 2-8 ml / min, and the inert gas flow rate was 90-120 ml / min. The temperature was kept constant. WO3 and Se underwent a chemical reaction to form a single layer of WSe2 single crystal on the gold foil. The growth time was 1-3 minutes. After the growth was completed, the temperature was rapidly reduced.

[0013] 2) The separation of the single-layer WSe2 single crystal from the gold foil substrate is achieved by electrochemical gas bubbling method. A polymethyl methacrylate / WSe2 composite film is obtained in the electrolyte. The composite film is supported by the target substrate to realize the transfer of WSe2. The polymethyl methacrylate protective film covering the surface of the WSe2 single crystal is dissolved by organic solvent.

[0014] This invention provides a method for improving the photoluminescence intensity of monolayer tungsten diselenide (WSe2) single crystals, belonging to the field of two-dimensional material optoelectronic device technology. Monolayer WSe2 single crystals grown by CVD are immersed in or spin-coated with 10%-40% concentration methanesulfonic acid aqueous solution. Photoluminescence tests were conducted on monolayer WSe2 single crystals immersed at different concentrations. It was found that the untreated monolayer WSe2 single crystal emitted the weakest light. With increasing concentration, the fluorescence intensity gradually increased, while the full width at half maximum (FWHM) decreased. Immersion in 40% methanesulfonic acid achieved a 4.55-fold fluorescence enhancement. Second harmonic distortion (HDC) tests were performed on the monolayer WSe2 single crystals before and after 40% HDC treatment, obtaining the relationship between the HDC signal and the polarization angle. Before and after HDC treatment, the HDC intensity of the monolayer WSe2 single crystals under parallel conditions, related to the polarization angle, exhibited a clear sixfold structure, indicating that HDC treatment at this dosage does not damage the crystal structure.

[0015] Monolayer WSe2 grown by CVD inevitably introduces intrinsic defects during fabrication due to inhomogeneities in carrier gas flow rate and temperature, leading to a decrease in optical performance. Modification with the strong organic acid methanesulfonic acid can fill these defects introduced during CVD growth. Photoluminescence testing shows that acid treatment enhances luminescence intensity and reduces the full width at half maximum (FWHM), indicating that the acid treatment repairs vacancy defects. Furthermore, by constructing a back-gate field-effect transistor, it is evident that the electrical performance is improved after acid treatment. Attached Figure Description

[0016] Figure 1 Samples treated by immersion under an optical microscope;

[0017] Figure 2 Samples treated by immersion under an optical microscope;

[0018] Figure 3 Results of photoluminescence testing on monolayer WSe2 single crystals soaked in different concentrations;

[0019] Figure 4 Results of second harmonic distortion (HDC) tests on monolayer WSe2 single crystals before and after treatment with 40% mass fraction acid;

[0020] Figure 5 Results of electrical performance testing of the original monolayer WSe2 single crystal;

[0021] Figure 6 The results of electrical performance tests on WSe2 single crystals soaked in 40% concentration by mass. Detailed Implementation

[0022] Example 1: A method for improving the photoluminescence intensity of monolayer tungsten diselenide (WSe2) single crystals.

[0023] The preparation method includes four parts: sample preparation is divided into preparation of monolayer WSe2 single crystal, transfer of monolayer WSe2 single crystal, acid treatment of monolayer WSe2 single crystal, and preparation of back gate field effect transistor.

[0024] 1. Preparation of monolayer WSe2 single crystals: Large-size WSe2 single crystals were rapidly prepared on a substrate using chemical vapor deposition. A horizontal muffle furnace was used as the reaction apparatus, and gold foil (Au) was selected as the growth substrate. The precursor reaction sources were metal oxide WO3 and chalcogen element nonmetal Se, which were placed in the high-temperature zone (850-900℃) and the low-temperature zone (350℃) respectively to participate in the reaction. During the heating stage, inert gas was introduced at a gas flow rate of 50-200 mL / min, and the temperature was increased at 15-40℃ / min to reach 800-900℃. During the material growth stage, hydrogen and inert gas were introduced, with hydrogen at 2-8 mL / min and inert gas at 90-120 mL / min while the temperature remained constant. WO3 and Se underwent a chemical reaction, forming a monolayer WSe2 single crystal on the gold foil. The growth time was 1-3 minutes, and the temperature was rapidly reduced after growth.

[0025] 2. Transfer of monolayer WSe2 single crystal: First, polymethyl methacrylate (PMMA) is uniformly spin-coated onto the gold foil surface on which the WSe2 single crystal is grown as a protective layer. The monolayer WSe2 single crystal is separated from the gold foil substrate using an electrochemical gas bubbling method. A PMMA / WSe2 composite film is obtained in an electrolyte. The composite film is then supported by another target substrate to achieve the transfer of WSe2. The PMMA protective film covering the surface of the WSe2 single crystal is dissolved and removed using an organic solvent to obtain a monolayer WSe2 single crystal.

[0026] 3. Acid treatment of single-layer WSe2 single crystal: Prepare a 10%-40% methanesulfonic acid aqueous solution, immerse the transferred WSe2 single crystal in the methanesulfonic acid aqueous solution for 10-60 minutes, blow dry the methanesulfonic acid solution adhering to the surface with high-purity nitrogen, and heat it on a heating table at a temperature of 90-140℃ for 5-10 minutes.

[0027] 4. Fabrication of Acid-Treatment Monolayer WSe2 Single-Crystal Back-Gate Field-Effect Transistors: Photoresist was uniformly spin-coated onto the acid-treated monolayer WSe2 single-crystal surface using a spin coater. The spin-coating steps were as follows: first, a low-speed spin coater was used at 500 rpm for 30 seconds, followed by a high-speed spin coater at 1500 rpm for 60 seconds. The spin-coated sample was then heated on a heating stage for 90 seconds at 120°C. Maskless photolithography was used to expose the monolayer WSe2 single-crystal surface. After development, a 40-60 nm thick gold electrode was deposited, and residual photoresist was removed using organic matter.

[0028] The acid treatment includes:

[0029] 1) Acid Treatment Preparation Method 1: Prepare a 20% (w / w) methanesulfonic acid solution. Weigh 79.9 g of deionized water using an electronic balance, and slowly add 20.1 g of methanesulfonic acid (99.5% (w / w) purchased from Aladdin). After the solution has cooled to room temperature, immerse the monolayer WSe2 single crystal transferred to the SiO2 / Si substrate in the prepared methanesulfonic acid solution for 20 minutes. Remove the monolayer WSe2 single crystal, dry it with high-purity nitrogen gas, and then heat it on a heating stage at 100°C for 5 minutes.

[0030] 2) Acid-treated preparation method two: Prepare a 20% (w / w) methanesulfonic acid solution. Weigh 79.9 g of deionized water using an electronic balance, and slowly add 20.1 g of methanesulfonic acid. After the solution cools to room temperature, it is then spin-coated using a spin coater. The spin-coating steps are as follows: First, use a pipette to drop the prepared methanesulfonic acid solution onto a single-layer WSe2 crystal. Spin coat at a low speed of 800 rpm for 8 seconds, then at a high speed of 1500 rpm for 30 seconds. After drying the droplets on the surface of the single-layer WSe2 crystal with high-purity nitrogen, place it on a heating stage at 100℃ and heat for 5 minutes.

[0031] 3) Acid Treatment Preparation Method Three: Prepare a 30% (w / w) methanesulfonic acid solution. Weigh 69.85 g of deionized water using an electronic balance, and slowly add 30.15 g of methanesulfonic acid. After the solution cools to room temperature, immerse the monolayer WSe2 single crystal transferred to the SiO2 / Si substrate in the prepared methanesulfonic acid solution for 20 minutes. Remove the monolayer WSe2 single crystal, dry it with high-purity nitrogen gas, and then heat it on a heating stage at 100℃ for 5 minutes.

[0032] 4) Acid Treatment Preparation Method Four: Prepare a 40% (w / w) methanesulfonic acid solution. Weigh 59.8 g of water using an electronic balance, and slowly add 40.2 g of methanesulfonic acid. After the solution cools to room temperature, immerse the monolayer WSe2 single crystal transferred to the SiO2 / Si substrate in the prepared methanesulfonic acid solution for 20 minutes. Remove the monolayer WSe2 single crystal, dry it with high-purity nitrogen gas, and then heat it on a heating stage at 100℃ for 5 minutes.

[0033] Sample characterization: Comparing samples treated with acid by immersion and spin-coating methods under an optical microscope, the sample immersed in the acid solution showed a smoother surface, while the spin-coated sample had residual acid droplets on its surface, such as… Figure 1 and Figure 2As shown in the figure, photoluminescence tests were performed on monolayer WSe2 single crystals immersed in different concentrations. It was found that the untreated monolayer WSe2 single crystals emitted the weakest light. With increasing concentration, the fluorescence intensity gradually increased, while the full width at half maximum (FWHM) decreased. Immersion in 40% methanesulfonic acid resulted in a 4.55-fold increase in fluorescence. Figure 3 As shown, second harmonic distortion (HHD) tests were performed on monolayer WSe2 single crystals before and after treatment with 40% mass fraction acid. The relationship between the HHD signal and polarization angle was obtained. Before and after acid treatment, the HHD intensity of the monolayer WSe2 single crystal under parallel conditions showed a clear six-fold structure related to the polarization angle, indicating that acid treatment at this dosage does not damage the crystal structure. Figure 4 As shown.

[0034] Electrical performance testing: Electrical performance tests were performed on the original monolayer WSe2 single crystal and the WSe2 single crystal after immersion in 40% concentration (by mass). Figure 5 and Figure 6 As shown, the drain current of WSe2 is significantly increased after acid treatment, indicating improved electrical performance.

[0035] Monolayer WSe2 grown by CVD inevitably introduces intrinsic defects during fabrication due to inhomogeneities in carrier gas flow rate and temperature, leading to a decrease in optical performance. Modification with the strong organic acid methanesulfonic acid can fill these defects introduced during CVD growth. Photoluminescence testing shows that acid treatment enhances luminescence intensity and reduces the full width at half maximum (FWHM), indicating that the acid treatment repairs vacancy defects. Furthermore, by constructing a back-gate field-effect transistor, it is evident that the electrical performance is improved after acid treatment.

Claims

1. A method for improving the photoluminescence intensity of a single-layer tungsten diselenide (WSe2) monocrystalline crystal, characterized in that: 1) Soak the monolayer WSe2 single crystal in a 10%-40% (w / w) methanesulfonic acid aqueous solution for 10-60 minutes; Alternatively, it can be spin-coated onto the surface of a single-layer WSe2 monocrystalline crystal using a spin coater; 2) Dry with nitrogen gas; 3) Place it on a heating table with a temperature of 90-140℃ and heat for 5-10 minutes.

2. The method for improving the photoluminescence intensity of a single-layer tungsten diselenide (WSe2) single crystal according to claim 1, characterized in that: The concentration of the methanesulfonic acid aqueous solution is 20%.

3. The method for improving the photoluminescence intensity of a single-layer tungsten diselenide (WSe2) single crystal according to claim 2, characterized in that: The spin coating is performed at a speed of 500 rpm for a spin coating time of 30 seconds; or at a high speed of 1500 rpm for a spin coating time of 60 seconds.

4. The method for improving the photoluminescence intensity of a single-layer tungsten diselenide (WSe2) monocrystalline crystal according to claim 1, 2, or 3, characterized in that: The monolayer WSe2 single crystal is prepared by the following method: 1) A horizontal muffle furnace was used as the reaction apparatus, and gold foil was selected as the growth substrate. The precursor reaction sources were metal oxide WO3 and chalcogen element nonmetal Se, which were placed in a high-temperature zone of 850-900℃ and a low-temperature zone of 350℃, respectively, to participate in the reaction. During the heating stage, inert gas was introduced at a gas flow rate of 50-200 ml / min, and the temperature was increased at 15-40℃ / min to reach 800-900℃. When the material entered the growth stage, hydrogen and inert gas were introduced. The hydrogen flow rate was 2-8 ml / min, and the inert gas flow rate was 90-120 ml / min. The temperature was kept constant. WO3 and Se underwent a chemical reaction to form a single layer of WSe2 single crystal on the gold foil. The growth time was 1-3 minutes. After the growth was completed, the temperature was rapidly reduced. 2) The separation of the single-layer WSe2 single crystal from the gold foil substrate is achieved by electrochemical gas bubbling method. A polymethyl methacrylate / WSe2 composite film is obtained in the electrolyte. The composite film is supported by the target substrate to realize the transfer of WSe2. The polymethyl methacrylate protective film covering the surface of the WSe2 single crystal is dissolved by organic solvent.

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