Near-infrared nonlinear absorption adjustable Nd-doped molybdenum disulfide and preparation method thereof

Abstract

The invention provides Nd-doped molybdenum disulfide with adjustable near-infrared nonlinear absorption and a preparation method of the Nd-doped molybdenum disulfide, belongs to the technical field of nano materials, and aims to solve the technical problem of poor near-infrared photoelectric property of MoS2. The preparation method of the Nd-doped molybdenum disulfide comprises the following steps: dissolving a molybdenum source, a sulfur source, a neodymium source and a reducing agent into a solvent to prepare a reaction solution, and performing solvothermal reaction to prepare the Nd-doped molybdenum disulfide nano material. The prepared MoS2 nanosheet is rich in VS, and the band gap of the MoS2 nanosheet is reduced to 0.75 eV and is obviously lower than the standard band gap (1.28-1.98 eV) of intrinsic MoS2, so that the nonlinear optical response at the wavelength of 1064 nm is successfully realized, the SA effect is shown, and the MoS2 nanosheet can be applied to devices such as a saturable absorber, an optical switch and a laser mode locker. Furthermore, by introducing Nd < 3 + > for doping, controllable conversion of nonlinear absorption behavior from SA to RSA along with light intensity change is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of nanomaterials, and particularly relates to a molybdenum disulfide. BACKGROUND

[0002] Nonlinear optical (NLO) materials are one of the key factors to promote the continuous development of optoelectronic technology, and have wide application prospects in the fields of pulsed laser generation, optical limiting, multi-photon pumping and photoelectric detection. Among them, the nonlinear absorption (nonlinear optical absorption refers to the phenomenon that the absorption characteristics (such as absorption coefficient, transmittance) of a material change with the light intensity under strong light (such as laser) irradiation) effect as the core physical mechanism, which shows the characteristics that the absorption coefficient of the material changes with the intensity of the incident light, is the basis for realizing the functions of light modulation and limiting. Among various NLO materials, two-dimensional materials show significant advantages due to their unique electronic structure and optical properties. Transition metal dichalcogenides (TMDCs) such as MoS2 have attracted much attention in nanophotonic devices due to their band gap that can be tuned from several eV to zero point several eV. However, the intrinsic band gap of MoS2 (about 1.28-1.98 eV) limits its light absorption ability in the near-infrared (NIR) region with a wavelength greater than 1000 nm, which seriously hinders its photoelectric application in this wavelength range. Therefore, introducing defects or implementing element doping to regulate its energy band structure has become an important way to expand its functions. Therefore, developing an accurate defect and doping synergistic regulation strategy is of great significance to improve the nonlinear optical performance of MoS2 in the near-infrared region.

[0003] In 2024, Chen et al. prepared W-doped MoS2 films with sulfur-vacancy-rich on quartz substrates by magnetron sputtering technology. Z-scan test results show that all W-doped samples exhibit significant RSA effect and excellent optical limiting performance, with nonlinear absorption coefficient as high as 6.7×10 -5 m / W, and the initial threshold is as low as 0.07 J / cm 2 , showing good application potential.

[0004] In 2024, Ducut et al. studied the influence of Cu doping on the structure, electronic and optical properties of MoS2 based on density functional theory (DFT) calculations. The study found that Cu doping introduces midgap states, resulting in a decrease in the band gap, and all optical responses peak in the ultraviolet-violet region, indicating its application value in nanophotonic devices.

[0005] In 2024, Abith et al. studied the nonlinear optical behavior of silver-modified reduced graphene oxide-MoS2 (Ag-rGO-MoS2) nanocomposites using a Q-switched Nd:YAG nanosecond pulsed laser through Z-scan technique. At low laser intensity, the material exhibited SA, attributed to the localized surface plasmon resonance effect of silver ions; while at high light intensity, it turned into RSA, which was due to the two-photon absorption process. The composite material showed excellent nonlinear response in a wide intensity range, suitable for applications such as optical switches and optical limiters.

[0006] In 2023, Zaimia et al. studied the effect of Ni doping on the structure, morphology, and optical properties of MoS2 nanosheets, and analyzed its nonlinear absorption and nonlinear refractive characteristics using the compact density matrix method. Interband and intraband transitions were considered as the main reason for the significant change in optical properties, and the results showed that Ni-doped MoS2 was suitable for optoelectronic devices.

[0007] In 2017, Li et al. achieved rare earth Eu-doped MoS2 through a hydrothermal method, which doubled the photoluminescence intensity and improved the crystallinity and electrical properties of the material, as well as the photovoltaic properties of the MoS2-Si heterojunction.

[0008] In 2019, Maddi et al. used femtosecond pulsed laser deposition (fs-PLD) technology to prepare Yb 3+ doped MoS2 films on a silicon substrate, achieving photoluminescence in the near-infrared band at 1064 nm. Open-aperture Z-scan tests showed that Yb 3+ doping increased the saturation light intensity of the material from (399±20) GW / cm 2 to (882±58) GW / cm 2 .

[0009] In 2025, Ding et al. prepared Er-doped MoS2 through chemical vapor deposition (CVD), found that it enhanced the photoluminescence intensity, and improved the optoelectronic performance of field-effect transistors.

[0010] In 2016, Meng et al. reported that Er 3+ doping could improve the carrier mobility and I-V characteristics of MoS2 films, and enhance the light absorption and luminescence intensity in the visible region at room temperature.

[0011] In 2021, Li et al. prepared Sm-doped MoS2 through CVD method, found that it could cause a positive shift in the threshold voltage of field-effect transistors, and increase the on-off ratio by 500%, indicating that Sm played a p-type doping role and effectively modulated the electronic structure of the material.

[0012] Most of the current researches focus on the regulation of a single type of defect or doping strategy on the performance of MoS2, and the synergistic effect of multiple modification methods has not been fully explored to achieve high-efficiency photoelectric performance improvement in a wider spectral range. It is difficult to break through the intrinsic band gap limitation by using a single regulation method, and it is impossible to realize wide-spectrum and high-efficiency light absorption and utilization from visible to near-infrared. Therefore, developing a defect and doping multi-element synergistic regulation method has become a key direction for further improving the near-infrared photoelectric performance of MoS2. SUMMARY

[0013] In view of the above technical problems, the present application provides a near-infrared nonlinear absorption controllable Nd-doped molybdenum disulfide and a preparation method thereof. The prepared MoS2 nanosheet is rich in VS, and its band gap is reduced to 0.75 eV, which is significantly lower than the standard band gap (1.28-1.98 eV) of intrinsic MoS2, thereby successfully realizing nonlinear optical response at a wavelength of 1064 nm and exhibiting SA effect, which can be applied to devices such as saturable absorber, optical switch and laser mode locker. Further, by introducing Nd 3+ doping, the controllable conversion of nonlinear absorption behavior from SA to RSA with light intensity change is realized.

[0014] In order to achieve the above purpose, the technical scheme of the present application is as follows: A near-infrared nonlinear absorption controllable Nd-doped molybdenum disulfide, and a preparation method thereof, the preparation method comprising the following steps: dissolving a molybdenum source, a sulfur source, a neodymium source and a reducing agent into a solvent to prepare a reaction solution, and preparing the Nd-doped molybdenum disulfide nanomaterial through a solvent thermal reaction.

[0015] Preferably, the molybdenum source is one or more than two of molybdenum trioxide, ammonium molybdate, ammonium molybdate tetrahydrate, sodium molybdate, potassium molybdate, ammonium paramolybdate, sodium paramolybdate, potassium paramolybdate, molybdenum acetate, molybdenum oxalate, molybdenum acetylacetone, and molybdenum phosphide.

[0016] Preferably, the sulfur source is one or more than two of thioacetamide, thiourea, sulfur powder, carbon disulfide, sodium sulfide, potassium sulfide, ammonium tetrathiomolybdate, butyl mercaptan, dimethyl sulfoxide and potassium thiocyanate.

[0017] Preferably, the neodymium source is neodymium chloride, neodymium nitrate or neodymium acetate.

[0018] Preferably, the reducing agent is one or more than two of urea, limonene and vitamin C.

[0019] Preferably, the solvent is water or / and ethanol.

[0020] Preferably, the mass ratio of the molybdenum source, the sulfur source and the reducing agent is 1:(1-3):(2-4).

[0021] Preferably, the amount of substance of the neodymium source is 0.05-0.5% of the molybdenum source, and the ratio of the molybdenum source to the solvent is 1-5 g / mL.

[0022] Preferably, the temperature of the solvothermal reaction is 180-240℃, and the time is 10-24h.

[0023] Advantages of the present application: (1) The MoS2 nanosheet prepared by the present application is rich in VS, and its band gap is reduced to 0.75 eV, which is significantly lower than the standard band gap (1.28-1.98 eV) of intrinsic MoS2, thereby successfully realizing nonlinear optical response at a wavelength of 1064 nm and exhibiting SA effect, which can be applied to saturable absorber, optical switch and laser mode locker and the like.

[0024] (2) The present application realizes the controllable conversion of nonlinear absorption behavior from SA to RSA by introducing Nd 3+ doping. The conversion mechanism is due to the synergistic coupling of Nd 3+ discrete energy levels and VS defect states, wherein the SA behavior dominated by ground state absorption is dominant at low light intensity, and the RSA effect dominated by excited state absorption (ESA) is dominant at high light intensity. This light intensity-dependent SA-RSA switching characteristic can be used to construct fast optical switches, optical limiters, optical pattern memories and even all-optical logic gates and the like. At the same time, Nd 3+ doping also significantly enhances the RSA effect of the material at 532 nm, further expanding its optical limiting application in the visible light region.

[0025] (3) The present application fully demonstrates the potential of Nd 3+ doped MoS2 nanosheet in wide-spectrum optoelectronic functional devices, including ultrafast laser modulation, high-performance optical limiting and phase modulation, etc., which provides a new way for the development of multi-dimensional synergistic regulation of nonlinear optical properties of two-dimensional materials. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed in the following embodiment or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0027] Figure 1 Preparation steps of rare earth Nd-doped MoS2 nanosheet.

[0028] Figure 2Fig. 1 (a) Atomic configuration schematic diagram of Nd:MoS2crystal structure at different angles, (b, c) SEM images of MoS2(comparative example 1), (d, e) SEM images of Nd:MoS2(embodiment 4).

[0029] Figure 3 Fig. 2 (a, b) Atomic force microscope images of MoS2(comparative example 1), Nd:MoS2(embodiment 4) nanosheets, (c, d) Thickness of MoS2(comparative example 1), Nd:MoS2(embodiment 4).

[0030] Figure 4 Fig. 3 (a-c) TEM images and lattice fringe spacing maps of MoS2(comparative example 1) nanosheets; (d-f) TEM images and lattice fringe spacing maps of Nd:MoS2(embodiment 4) nanosheets; (g) Mo, S, Nd element energy spectrum area distribution maps of Nd:MoS2nanosheets in (d).

[0031] Figure 5 Fig. 4 (a) XRD diffraction patterns of MoS2(comparative example 1), Nd:MoS2(embodiment 4) nanosheets; (b) Raman spectra of MoS2, Nd:MoS2nanosheets; (c) XPS spectra of MoS2, Nd:MoS2; (d-f) XPS maps of S 2p orbit (d), Nd 3d orbit (e), Mo 3d (f) core level of Nd:MoS2nanosheets.

[0032] Figure 6 Fig. 5 (a) UV-Vis absorption spectra of MoS2(comparative example 1), Nd:MoS2(embodiment 4) nanosheets; (b) PL intensity comparison of MoS2, Nd:MoS2nanosheets; (c) Tauc plots of MoS2, Nd:MoS2nanosheets; (d, e) Projected band structures and density of states (Fermi level set as valence band top) of MoS2, Nd:MoS2nanosheets, respectively.

[0033] Figure 7 Fig. 6 (a-d) OAZ scanning curves of MoS2, Nd:MoS2nanosheets under excitation wavelength of 532 nm and excitation energy of 1 µJ~5 µJ; (e, f) OAZ scanning curves of MoS2, Nd:MoS2nanosheets under excitation wavelength of 1064 nm and excitation energy of 1 µJ~4 µJ (the dots in the figure are experimental data; the solid line is the curve fitted according to the experimental data using the model).

[0034] Figure 8(a, b) The relationship between the output pulse energy and the input pulse energy of MoS2, Nd:MoS2 nanoplatelets at an excitation wavelength of 532 nm and an excitation energy of 1 µJ~5 µJ; (c, d) The relationship between the output pulse energy and the input pulse energy of MoS2, Nd:MoS2 nanoplatelets at an excitation wavelength of 1064 nm and an excitation energy of 1 µJ~4 µJ (black dotted line: the output pulse energy and the input pulse energy are in a linear relationship; the colored dotted line is a trajectory line drawn according to the experimental results).

[0035] Figure 9 Schematic diagram of carrier dynamics in MoS2 and Nd:MoS2 nanoplatelets.

[0036] Figure 10 (a, b) The relationship between the normalized transmittance and the input pulse energy of MoS2, Nd:MoS2 nanoplatelets at an excitation wavelength of 532 nm and an excitation energy of 1 µJ~5 µJ; (c, d) The relationship between the normalized transmittance and the input pulse energy of MoS2, Nd:MoS2 nanoplatelets at an excitation wavelength of 1064 nm and an excitation energy of 1 µJ~4 µJ (dotted line: the starting point of MoS2, Nd:MoS2 nanoplatelet SA, RSA).

[0037] Figure 11 (a, b) Two-dimensional diagram of the pump-probe time delay corresponding to MoS2, Nd:MoS2 nanoplatelets; (c, d) Carrier decay dynamics corresponding to MoS2, Nd:MoS2 nanoplatelets at the corresponding probe peak wavelength, the solid line is the fitting curve. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0039] Embodiment 1 A near-infrared nonlinear absorption controllable Nd-doped molybdenum disulfide, the preparation schematic diagram is as shown in Figure 1 The specific preparation method comprises the following steps: (1) 3.4 g of MoO3, 2.4 g of thioacetamide, 2.6 g of thiourea and 6.15 g of urea are weighed, dissolved in 100 mL of deionized water, magnetically stirred for 3 h, and a uniform precursor solution W1 is formed.

[0040] (2) Add 0.01 g of NdCl3 to W1 and continue stirring for 3 h to obtain mixed solution W2; (3) W2 is placed in a high-pressure reactor and kept at 200°C for 12 hours for hydrothermal reaction; (4) After the reaction is complete, allow the mixture to cool naturally, collect the product, and wash it three times by centrifugation with water and ethanol respectively. (5) The cleaned sample was pre-frozen and freeze-dried for 24 hours to obtain Nd:MoS2 nanosheet powder.

[0041] Examples 2-6 The difference from Example 1 lies in the amount of NdCl3 used in step (2) and the hydrothermal time in step (3). The specific process parameters are shown in Table 1: Table 1. Summary of process parameters in Examples 1-6 Comparative Example (1) Weigh 3.4 g MoO3, 2.4 g thioacetamide, and 2.6 g thiourea / urea, dissolve them in 100 mL of deionized water, and stir magnetically for 3 h to form a homogeneous precursor solution W1.

[0042] (2) W1 is placed in a high-pressure reactor and kept at 200°C for 12 hours for hydrothermal reaction; (3) After the reaction is complete, allow the mixture to cool naturally, collect the product, and wash it three times by centrifugation with water and ethanol respectively. (4) The cleaned sample was pre-frozen and freeze-dried for 24 hours to prepare Nd-free samples. 3+ Doped MoS2.

[0043] This invention utilizes a simple hydrothermal method to prepare a product containing V s Nd:MoS2 nanosheets ( Figure 2 a). SEM showed that MoS2 is a flower-like structure composed of nanosheets (comparative example). Figure 2 b and 2c), while Nd:MoS2 formed a spherical structure composed of nanosheets (Example 4, Figure 2 The results (d and 2e) are attributed to the alteration of the assembly structure of MoS2 layered nanosheets by rare-earth Nd doping, demonstrating the successful preparation of MoS2 and Nd:MoS2 nanosheets. Atomic force microscopy revealed that the radius of the MoS2 and Nd:MoS2 nanosheets was approximately 200 nm, and the nanosheet thickness was approximately 2.4 nm, indicating that the MoS2 and Nd:MoS2 nanosheets had approximately 4 layers. Figure 3 ).

[0044] Figure 4 TEM images of MoS2 (Comparative Example 1) and Nd:MoS2 (Example 4) nanosheets are presented. From Figure 4As can be seen in (a-c), the MoS2 nanosheets present a layered structure, and the MoS2 nanosheets have clear lattice fringes with a lattice spacing of about 0.283 nm. From Figure 4 As can be seen in (d-f), the morphology of the Nd:MoS2 nanosheets is changed compared to the MoS2 nanosheets, but the Nd:MoS2 still has clear lattice fringes with a lattice spacing of 0.289 nm, which may be because the atomic radius of Nd is larger than that of Mo, and the doping causes lattice distortion to change the average distance between adjacent atoms, thereby causing slight expansion of the entire crystal structure. From energy dispersive X-ray (EDX) spectral analysis, the Mo, S and Nd elements are uniformly distributed in the Nd:MoS2 nanosheets, further proving the successful doping of Nd. 3+

[0045] Figure 5 (a) gives the XRD diffraction patterns of the MoS2 (comparative example 1) and Nd:MoS2 (example 4) nanosheets, and from the figure it can be seen that characteristic peaks appear at 14.28°, 34.18° and 57.2°, corresponding to the (002) crystal plane, (100) crystal plane and (110) crystal plane of MoS2, respectively, wherein the (002) crystal plane belongs to interlayer diffraction along the c-axis direction, reflecting the periodicity of the S-Mo-S layered stacking and the interlayer distance thereof, indicating that the MoS2 nanosheets have a good layered structure, the (100) crystal plane is perpendicular to the c-axis and belongs to diffraction in the a-b plane, showing the atomic arrangement in the MoS2 monolayer, which is the repeating unit of S-Mo-S along the a-axis direction, which is consistent with the TEM and SEAD results, and therefore, the 2H phase MoS2 nanosheets with a hexagonal crystal structure are successfully prepared by the hydrothermal method. Compared with the MoS2 nanosheets, the XRD diffraction peaks of the Nd:MoS2 nanosheets do not change significantly, indicating that the Nd doping does not change the crystal structure of the MoS2 nanosheets, and the Nd atoms exist in the MoS2 nanosheets in the form of substitutional doping. From Figure 5 b, for the MoS2 nanosheets, two typical characteristic peaks appear at 373.69 cm -1 and 400.47 cm -1 , corresponding to E21g and A1g vibrations, respectively. The E21g characteristic peak is due to the symmetric stretching vibration of the Mo-S bond, i.e. the symmetric stretching movement of the Mo atoms and S atoms in the a-b plane, and the A1g characteristic peak is derived from the out-of-plane symmetric vibration of the S atoms (along the c-axis direction), i.e. the two layers of S atoms move in the direction perpendicular to the a-b plane in opposite directions, while the Mo atoms remain stationary, which are characteristic peaks of the 2H phase MoS2 of hexagonal system. Compared with the MoS2 nanosheets, the Nd:MoS2 nanosheets have characteristic peaks at 373.76 cm -1 and 401.03 cm -1 ​The appearance of E21 g and A1 g vibration characteristic peaks, showing red shift phenomenon, is because Nd:MoS2 is p-type doped during the growth process. Since the electronegativity of Mo and Nd is similar, and the atomic radius difference is small, Nd doping substitutes Mo. Since Nd ion is a cation, and MoS2 itself is an n-type semiconductor (with free electrons in the conduction band), the doping of Nd 3+ will introduce holes (i.e. positive charge carriers), thereby moving the Fermi level down to approach the valence band top. This movement of the Fermi level makes the material exhibit p-type doping characteristics, resulting in hexagonal lattice distortion and increased hole injection, which increases the carrier concentration, wherein S 2- acts as an acceptor and donor Nd 3+ causes tensile stress, reducing electron-phonon scattering. In summary, Nd doping causes changes in charge and strain of Nd:MoS2 nanosheets, resulting in red shift phenomenon, which is consistent with the results of p-type doping of Nb-doped MoS2 in other studies.

[0046] Figure 5 (c-f) gives the X-ray photoelectron spectroscopy (XPS) of MoS2, Nd:MoS2 nanosheets, which is obtained by Figure 5 c can be seen that, compared with MoS2, the Mo and S characteristic peaks in Nd:MoS2 nanosheets are shifted, which is due to the change in nanosheet lattice spacing caused by Nd doping. In the S core level spectrum peaks of MoS2 and Nd:MoS2 nanosheets (d), Figure 5 both the characteristic peaks located at about 163.8 eV and 164.9 eV are observed, which correspond to bridging disulfide type sulfur (S2 2- ) or unsaturated sulfur vacancies (Vs), again confirming the presence of sulfur vacancies in both materials. In addition, the S 2p spectrum of MoS2 shows double peaks at 162.14 eV (S 2p3 / 2) and 163.37 eV (S 2p1 / 2), which belong to S 2- species in the basal plane MoS2. Notably, the S 2p characteristic peaks (S 2p3 / 2 and S 2p1 / 2) of Nd:MoS2 nanosheets are shifted by about 0.1 eV to lower binding energy compared with MoS2. Similarly, the Mo 3d high-resolution XPS spectrum of Nd:MoS2 (e) clearly observes Nd characteristic peaks at 975.86 eV and 998.43 eV, again confirming the successful doping of Nd. Figure 5 Figure 5 f shows that the Mo 3d spectrum of MoS shows characteristic peaks at 229.24 eV (Mo 3d5 / 2) and 232.65 eV (Mo 3d3 / 2) belonging to Mo 4+ -S bond, while the characteristic peak at 236.0 eV corresponds to Mo 6+ ​Notably, the characteristic Mo 3d peaks (Mo 3d5 / 2 and Mo 3d3 / 2) of Nd:MoS3 / 2 nanosheets also shifted by approximately 0.1 eV towards lower binding energies. These results indicate that Nd... 3+ The doping of S and Mo causes the core energy level spectral peaks of both S and Mo to shift towards lower binding energies, which may be due to the transformation of the material from n-type conductivity to p-type conductivity caused by Nd doping.

[0047] Figure 6 a shows the UV-Vis absorption spectra of MoS2 (Comparative Example 1) and Nd:MoS2 (Example 4) nanosheets. Compared with MoS2, Nd:MoS2 exhibits significantly enhanced background absorption across the entire spectral range. This enhancement may stem from two factors: firstly, the introduction of Nd during the hydrothermal synthesis process. 3+ This could lead to distortion of its octahedral crystal field, thereby perturbing the continuity of the two-dimensional nanosheet structure, which may contribute additional light scattering at the Rayleigh scattering scale; on the other hand, Nd 3+ The introduction of new electronic energy levels may increase the carrier concentration, thereby enhancing the overall light absorption capacity of the material, which is consistent with the observations of Raman spectroscopy. Figure 6 b shows the room-temperature photoluminescence (PL) spectra of MoS2 and Nd:MoS2 nanosheets measured using a 365 nm laser as the excitation source. The PL intensity of Nd:MoS2 nanosheets is significantly enhanced compared to MoS2, with an enhancement factor nearly twice that of MoS2. This indicates that Nd... 3+ The doping of Nd2 effectively improves the light absorption efficiency and radiative recombination efficiency of MoS2 nanosheets. 3+ The introduced energy level can serve as a highly efficient excitation center, effectively capturing excited state energy and promoting photon emission through radiative recombination, thereby significantly improving the overall luminescence performance of the material. Figure 6 c gives the band gap values ​​of MoS2 and Nd:MoS2 nanosheets ( E g ). Nd 3+ Doping makes MoS2 E g The voltage decreased from 0.75 eV to 0.45 eV for Nd:MoS2. It is noteworthy that the standard defect-free 2H phase tetralayer MoS2... E g Approximately 1.28 eV. The undoped MoS2 nanosheets in this paper... E g (≈0.75eV) is significantly smaller than this value, which is likely due to the V introduced during the hydrothermal synthesis process. S Among them, V STypically manifested as shallow donors or deep level traps, these can introduce defect states into the band gap. The presence of these defect states may cause electrons that originally belonged to the conduction band to be localized, thereby reducing the effective band gap of the material.

[0048] To delve deeper into Vs and Nd 3+ To investigate the effect of doping on the electronic structure of MoS2, we performed density functional theory (DFT) calculations as follows: Figure 6 As shown in figure d, the introduction of Vs significantly alters the conduction band (CB) and valence band (VB) edges of MoS2, creating a new defect level between the conduction band minimum (CBM) and the Fermi level, leading to changes in theoretical calculations. E g The voltage decreased from 1.3 eV in the defect-free system to 0.37 eV in the Vs-containing system. Further doping of Nd into Vs-containing MoS2... 3+ back( Figure 6 e), Theoretical Calculation E g The voltage was further reduced to 0.29 eV, and a new energy level was observed. It should be noted that the theoretical calculations yielded... E g The values ​​(0.37 eV and 0.29 eV) are generally lower than the experimental values ​​(0.75 eV and 0.45 eV), mainly due to the generalized gradient approximation functional (PBE) used in the calculations, which generally underestimates the semiconductor. E g The systematic error. Furthermore, density of states (DOS) calculations show that Nd... 3+ Doping introduces additional electronic states that may participate in accommodating photogenerated electrons, thereby enhancing the light absorption process.

[0049] Figure 7 The Z-scan curves of MoS2 and Nd:MoS2 nanosheets at pulse energies of 532 nm, 1 µJ, and 2 µJ are presented. At 1 µJ, both samples exhibit a significant oscillation-modulation (SA) effect. This is mainly due to the photon energy (hν≈2.33 eV) being higher than the band gap of the samples (Eg≈0.75 eV and 0.45 eV), which excites valence band (VB) electrons to fill the conduction band (CB) energy level. According to the Pauli exclusion principle, when the excited state is occupied by electrons to a certain extent, the ground state absorption becomes saturated, leading to the SA effect. As the laser energy increases to 2 µJ, the SA modulation amplitude at the focal point is further enhanced. Figure 7(b-d) give the Z-scan curves of MoS2 and Nd:MoS2 at 532 nm under 3, 4, 5 µJ energy. When the energy increases to 3 µJ, the SA effect of MoS2 and Nd:MoS2 weakens, and their normalized transmittance curves are greater than 1 (SA dominated) far from the focus (low light intensity region) and less than 1 (RSA dominated) near the focus (high light intensity region), showing a "peak-valley" feature, indicating that the NLA behavior of MoS2 and Nd:MoS2 changes from SA to RSA. When the energy further increases to 4 µJ and 5 µJ, the normalized transmittance in the focus region further decreases, the SA effect continues to weaken, and the RSA effect significantly enhances, and Nd:MoS2 exhibits stronger RSA characteristics than MoS2. Figure 7 e gives the Z-scan curves of MoS2 nanosheets at 1064 nm under 1-4 µJ energy. MoS2 nanosheets exhibit SA effect at all tested energies. It is worth noting that the corresponding photon energy (hv≈1.16 eV) at 1064 nm is lower than the band gap (Eg≈1.28 eV) of standard 2H phase MoS2, which does not theoretically satisfy the ground state absorption condition of Eg<hv. However, SA response is observed in the infrared band for the MoS2 nanosheets prepared by the present application, which is due to the significant reduction of the actual band gap (Eg≈0.75, Eg≈0.37 obtained by DFT calculation) caused by a large number of Vs and edge defects generated in the hydrothermal synthesis process. These defects introduce intermediate states in the band gap, making sub-band gap photon absorption (defect-assisted absorption) possible. Figure 7 f gives the Z-scan curves of Nd:MoS2 nanosheets at 1064 nm under 1-4 µJ energy. At 1-2 µJ energy, Nd:MoS2 exhibits similar SA effect as MoS2. However, when the energy increases to 3-4 µJ, MoS2 still exhibits SA, while Nd:MoS2 changes from SA to RSA. This indicates that Nd 3+ Doping effectively regulates the NLA response of MoS2 nanosheets and adds RSA behavior at higher light intensity.

[0050] Figure 8(a, b) show the output energy (Fout) as a function of the input energy (Fin) for MoS2, Nd:MoS2 nanosheets at 532 nm laser wavelength. In the lower input energy region, Fout of MoS2, Nd:MoS2 nanosheets increases linearly with Fin, i.e. the slope of Fin / Fout curve is greater than 1. This indicates that the normalized transmittance of the material increases with the increase of light intensity, which is a typical characteristic of SA effect. With the increase of input energy, the trend of Fout with Fin weakens and gradually changes into sub-linear growth, i.e. the slope of Fin / Fout curve is less than 1. This indicates that the normalized transmittance of the material decreases with the increase of light intensity, which corresponds to RSA effect. In addition, when the laser increases to a higher input energy, the Fin / Fout curve is no longer linear, and this nonlinear dependence further verifies that MoS2, Nd:MoS2 nanosheets have significant NLA characteristics at 532 nm wavelength. Figure 8 (c, d) show the output energy (Fout) as a function of the input energy (Fin) for MoS2, Nd:MoS2 nanosheets at 1064 nm laser wavelength. For MoS2 nanosheets, Fout increases linearly with Fin in the whole input energy range, indicating that its normalized transmittance monotonically increases with light intensity, only showing SA effect. For Nd:MoS2 nanosheets, linear growth is also observed in the lower input energy region, indicating the existence of SA effect. With the further increase of input energy, the trend of Fout with Fin weakens and changes into sub-linear growth (Fin / Fout<1), indicating that its normalized transmittance begins to decrease with the increase of light intensity, and RSA effect appears. The results show that Nd:MoS2 nanosheets not only exhibit SA effect, but also have RSA effect, and have strong nonlinear optical response and excellent light intensity-dependent light signal modulation ability.

[0051] Table 2 gives the third-order nonlinear parameters of MoS2, Nd:MoS2 nanosheets at 532 nm wavelength. As can be seen from the table, at 532 nm wavelength, whether the NLA response shows SA or RSA effect, Nd 3+ Doping significantly improves the saturation light intensity (I S ) of MoS2 nanosheets. To explain this phenomenon, the present application proposes a physical mechanism of doping effect represented by a four-level model based on the valence band (VB) and conduction band (CB) energy transfer of the electronic structure of MoS2 and Nd:MoS2 nanosheets Figure 9 ). The 4f electron orbit of Nd 3+ ion introduces new energy levels in the forbidden band of MoS2, significantly changing its electronic structure, and these new energy levels provide additional electron transition paths. Raman spectrum results show that Nd 3+MoS2 was p-doped, increasing the hole concentration (carrier concentration) in the material. Higher carrier concentration means more energy is needed to excite the same proportion of carriers, which can be one of the reasons for the higher I S of the doped samples. In addition, Nd 3+ doping also significantly increased the nonlinear absorption coefficient β of the material. For example, at 4µJ excitation energy, the nonlinear absorption coefficient β of MoS2 nanoplatelets was 7.4 x 10 -11 m / W, while that of Nd:MoS2 increased to 10 x 10 -11 m / W. At 5µJ excitation energy, the β of MoS2 was 8.5 x 10 - 11 m / W, while that of Nd:MoS2 increased to 12 x 10 -11 m / W. These results show that Nd ³+ doping effectively enhances the RSA capability of the material at high light intensity.

[0052] Table 3 gives the third-order nonlinear parameters of MoS2, Nd:MoS2 nanoplatelets at 1064 nm wavelength. At 1064 nm, 1µJ and 2µJ excitation energy, the NLA responses of MoS2, Nd:MoS2 nanoplatelets all exhibit SA effect, while Nd ³+ doping also significantly increased the I S . For example, at 1µJ, the I 17 of MoS2 was 2 x 10 -2 GW cm 18 , while that of Nd:MoS2 increased to 1 x 10 -2 GW cm 17 , about 5 times increase; at 2µJ, the I -2 of MoS2 was 2.5 x 10 18 GW cm -2 , while that of Nd:MoS2 increased to 3 x 10 3+ GW cm 3+ , about 12 times increase, with about 1 order of magnitude increase in the saturation intensity. This is consistent with the observation at 532 nm as mentioned above, further demonstrating the effect of doping on increasing the saturation threshold. When the excitation energy increased to 3µJ~4µJ, the NLA response of MoS2 still exhibited SA effect, while that of Nd:MoS2 changed to RSA effect. The persistent SA effect of MoS2 is due to its ground state absorption cross section area (ɕ1) being larger than the excited state absorption cross section area (ɕ2). The RSA effect of Nd:MoS2, on the other hand, is attributed to NdThe introduced discrete energy levels significantly increase the excited-state absorption cross section α2, where α2 > α1. These absorbance levels provide effective excited-state absorption channels, increasing the probability of excited-state absorption (ESA) and thus dominating the NLA response at high light intensities, leading to the transition from SA to RSA. This mechanism will be further verified in a later section with more in-depth spectroscopic characterization (transient absorption spectroscopy). The above results indicate that Nd... ³+ Doping can significantly improve the Ig of Nd:MoS2 nanosheets s This enhances its anti-saturation ability, strengthens the nonlinear absorption coefficient β under high light intensity of 532 nm, and reinforces the RSA effect; under high light intensity of 1064 nm, it induces the NLA response to switch from SA to RSA, expanding the nonlinear optical function of the material, indicating that Nd... ³+ Doping effectively modulates and significantly enhances the nonlinear optical absorption capability of Nd:MoS2 nanosheets.

[0053] Table 2. Third-order nonlinear parameters of MoS2 and Nd:MoS2 nanosheets at 532 nm wavelength. Table 3. Third-order nonlinear parameters of MoS2 and Nd:MoS2 nanosheets at 1064 nm wavelength. Figure 10 The relationship between the normalized transmittance of MoS2 and Nd:MoS2 nanosheets and the input laser pulse energy is shown. For example... Figure 10 As shown in (a,b), in the visible light region (532 nm), when the input energy is low (1 µJ and 2 µJ), the normalized transmittance of both MoS2 and Nd:MoS2 nanosheets increases with increasing input energy and gradually deviates from a linear response, exhibiting obvious saturable absorber (SA) characteristics. This indicates that both MoS2 and Nd:MoS2 nanosheets have the potential to be used as saturable absorbers at a wavelength of 532 nm, suitable for devices such as mode-locked lasers, Q-switches, and optical switches / modulators. However, when the input energy increases to 3 µJ, 4 µJ, and 5 µJ, the normalized transmittance of MoS2 and Nd:MoS2 nanosheets shows a trend of first increasing (dominated by saturable absorption) and then decreasing (dominated by anti-saturable absorption). This indicates that the nonlinear absorption behavior of the materials has changed from SA to RSA. Notably, in the near-infrared region (1064 nm)... Figure 10 (c and 10d) As the input energy increases from low to high (1-4 µJ), MoS2 exhibits SA characteristics, while Nd:MoS2 nanosheets exhibit SA characteristics at low input energies but switch to RSA characteristics at high input energies. Therefore, the results indicate that due to Nd... 3+Doping, the controllable transition of Nd:MoS2nanosheets from SA to RSA has been extended from the visible region to the near-infrared region. The existence of SA effect makes this material show application prospects in optical limiters and laser pulse shaping, while the transition of SA-RSA shows application prospects in various optical devices such as fast optical switches, optical limiters, optical bistable devices, and even all-optical logic gates that require dynamic light control functions. In addition, the onset threshold is a key parameter for characterizing SA and RSA characteristics, which defines the critical light intensity at which the material transitions from linear absorption to significant nonlinear absorption behavior. For mode-locking applications, a lower SA onset threshold generally means that the device can function at a lower operating light intensity (e.g., to initiate mode-locking), which makes it one of the important indicators for evaluating performance quality. The above results show that although MoS2and Nd:MoS2nanosheets are in the same type of nonlinear absorption (NLA) response range, Nd:MoS2always exhibits a lower SA onset threshold than MoS2. This further confirms that Nd 3+ Doping effectively reduces the saturation threshold of the material, thereby enhancing its performance as a saturable absorber. Notably, at a wavelength of 1064 nm and an excitation energy of 3-4 µJ, MoS2nanosheets exhibit SA behavior, while Nd:MoS2nanosheets exhibit RSA behavior. Since the two materials are dominated by different NLA mechanisms at this stage, the corresponding critical threshold parameters (SA saturation threshold and RSA onset threshold) are not physically comparable. Therefore, no threshold comparison was made at this energy point.

[0054] To further characterize Nd 3+ The influence of doping on the nonlinear optical properties of MoS2nanosheets was tested using femtosecond time-resolved pump-probe technology to explore the ultrafast NLO response and carrier dynamics of MoS2and Nd:MoS2nanosheets. Figure 11 (a, b) give the three-dimensional TA spectra of MoS2and Nd:MoS2nanosheets obtained under 400 nm femtosecond laser pulse excitation. A significant positive absorption signal (Δ A >0) was observed near a wavelength of about 600 nm, which corresponds to ESA. The ESA observed in this experiment is mainly due to the transition of photo-generated carriers within the CB or to higher energy levels. The transient absorption kinetic decay curves obtained in the experiment were analyzed using a double exponential function, and the fitting results are shown in Figure 11 (c, d) show that the fast relaxation time constant τ1≈2.2 ps and the slow relaxation time constant τ2≈24.3 ps for MoS2nanosheets; the τ 1≈1.2 ps, τ2≈84.8 ps. The fast relaxation process (τ1) corresponds to the fast trapping of carriers by defect states or trap centers in the material, and the slow relaxation process (τ2) corresponds to the recombination of photo-generated electron-hole pairs (including radiative and non-radiative recombination). In the above results, τ1 is shortened (2.2 ps→1.2 ps), indicating that Nd 3+ The ions introduce additional effective trap centers in MoS2, significantly accelerating the initial trapping process of carriers, and τ2 is significantly prolonged (24.3 ps→84.8 ps), indicating that the carriers trapped by defects / traps have a longer lifetime. These long-lived carriers have a higher probability of being re-excited to a higher excited state by pump light (i.e., ESA occurs). Therefore, the excited state absorption process in Nd:MoS2 is enhanced, meaning that its excited state absorption cross section σ2 increases relative to the ground state absorption cross section σ1, which is a key factor leading to the enhancement of RSA behavior. At the same time, the dominant path of carrier recombination may also change due to the presence of trap states. In summary, the TA measurement results directly reveal that the Nd 3+ The significant regulation of defect states introduced by doping on the carrier dynamics and nonlinear absorption characteristics of MoS2 nanosheets provides key evidence for the change in NLO performance.

[0055] Embodiment 7 A Nd-doped molybdenum disulfide with tunable near-infrared nonlinear absorption, the specific preparation method comprising the following steps: (1) Take 5 g of MoO3, 2.4 g of thioacetamide, 2.6 g of thiourea, and 10 g of urea, dissolve in 100 mL of deionized water, and magnetically stir for 3 h to form a uniform precursor solution W1.

[0056] (2) Add 0.01 g of NdCl3 to W1 and continue stirring for 3 h to obtain a mixed solution W2; (3) Put W2 into a high-pressure reaction kettle and perform hydrothermal reaction at 180°C for 24 h; (4) After the reaction is completed, naturally cool down, collect the product, and centrifugally wash with water and ethanol for 3 times, respectively; (5) The washed sample is pre-frozen and freeze-dried for 24 hours to obtain Nd:MoS2 nanosheet powder.

[0057] Embodiment 8 A Nd-doped molybdenum disulfide with tunable near-infrared nonlinear absorption, the specific preparation method comprising the following steps: (1) Take 1 g of MoO3, 3 g of thioacetamide, and 4 g of urea, dissolve in 100 mL of deionized water, and magnetically stir for 3 h to form a uniform precursor solution W1.

[0058] (2) 0.01 g of NdCl3 was added into W1, and stirring was continued for 3 h to obtain a mixed solution W2; (3) W2 was loaded into a high-pressure reaction kettle, and hydrothermal reaction was carried out at 240°C for 10 h; (4) After the reaction was completed, natural cooling was carried out, and the product was collected and washed with water and ethanol by centrifugation for 3 times, respectively; (5) The sample after washing was pre-frozen and freeze-dried for 24 h to obtain Nd:MoS2 nanosheet powder.

[0059] The above merely describes preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a near-infrared nonlinear absorption controllable Nd-doped molybdenum disulfide, characterized in that, The method comprises the following steps: The reaction solution is prepared by dissolving a molybdenum source, a sulfur source, a neodymium source and a reducing agent into a solvent, and the Nd-doped molybdenum disulfide nanomaterial is prepared through a solvothermal reaction.

2. The method for preparing Nd-doped molybdenum disulfide with tunable near-infrared nonlinear absorption according to claim 1, characterized in that, The molybdenum source is one or more than two of molybdenum trioxide, ammonium molybdate, ammonium molybdate tetrahydrate, sodium molybdate, potassium molybdate, ammonium dimolybdate, sodium dimolybdate, potassium dimolybdate, molybdenum acetate, molybdenum oxalate, molybdenum acetylacetone, and molybdenum phosphide.

3. The method for preparing Nd-doped molybdenum disulfide with tunable near-infrared nonlinear absorption according to claim 2, characterized in that, The sulfur source is one or more than two of thioacetamide, thiourea, sulfur powder, carbon disulfide, sodium sulfide, potassium sulfide, ammonium tetrathiomolybdate, butyl mercaptan, dimethyl sulfoxide and potassium thiocyanate.

4. The method for preparing the near-infrared nonlinear absorption controllable Nd-doped molybdenum disulfide according to claim 3, characterized in that, The neodymium source is one or more than two of neodymium chloride, neodymium nitrate and neodymium acetate.

5. The method for preparing the near-infrared nonlinear absorption controllable Nd-doped molybdenum disulfide according to claim 4, characterized in that, The reducing agent is one or more than two of urea, limonene and vitamin C.

6. The method of claim 5, wherein the method is characterized by: The solvent is water or / and ethanol.

7. The method for preparing the near-infrared nonlinear absorption controllable Nd-doped molybdenum disulfide according to any one of claims 1-6, characterized in that, The mass ratio of the molybdenum source, the sulfur source and the reducing agent is 1:(1-3):(2-4).

8. The method of claim 7, wherein the method is characterized by: The amount of substance of the neodymium source is 0.05-0.5% of the molybdenum source, and the ratio of the molybdenum source to the solvent is 1-5 g / mL.

9. The method of claim 1, wherein the method is characterized by: The temperature of the solvothermal reaction is 180-240 ℃, and the time is 10-24 h.

10. The Nd-doped molybdenum disulfide prepared by the method of any one of claims 1-9 has adjustable near-infrared nonlinear absorption.

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