A two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization and its preparation method

Abstract

The present invention discloses a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization and a preparation method thereof. The material consists of a two-dimensional heterojunction material and a hard substrate; the two-dimensional heterojunction material includes a single-layer two-dimensional transition metal chalcogenide and a single-layer two-dimensional transition metal chalcogenide alloy; there is no slip between the substrate and the two-dimensional heterojunction material; the interlayer twist angle between the single-layer two-dimensional transition metal chalcogenide and the single-layer two-dimensional transition metal chalcogenide alloy is 0.1-3° or 3-22°. The material is obtained by mechanically exfoliating the single-layer raw materials and stacking them on a hard substrate and then performing layer-by-layer annealing treatment. The material utilizes the double-well competition regulation mechanism of chalcogen atom vacancy defects and Moiré periodic potential between materials, and by controlling the interlayer twist angle of the heterobilayer, different localization of interlayer excitons and the technical effect of valley polarization are achieved, which can be used for the design and optimization of spin-valley optoelectronic devices for defect engineering.

Description

Technical Field

[0001] The present invention relates to a two-dimensional heterojunction material, and particularly to a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization and a preparation method thereof, belonging to the technical field of two-dimensional semiconductor preparation. Background Art

[0002] Structural defects in monolayer 2D TMDs play an important role in their electronic and optoelectronic properties. Various defect structures are prevalent in 2D TMDs synthesized using existing synthesis techniques and manufacturing tools. Especially in alloys, the most common defect in 2D TMDs is the chalcogen atom vacancy defect because they are easily lost during growth or post-treatment. In terms of electronic properties, in-situ substitution of sulfur vacancies with oxygen atoms can change the carrier type and concentration of the material. When Se vacancies and Pt atoms form a stable phase, a semiconductor-to-metal transition can even be achieved in the initially stoichiometric 2D PtSe2 layer. In terms of optoelectronic properties, chalcogen atom vacancy defects usually lead to defect-related energy states, resulting in the formation of defect-bound excitons, an extended exciton lifetime, and a wide range of photoluminescence (PL) energy emission (half-width > 100 meV). Therefore, understanding and controlling defects are of great significance for exploring and regulating the optoelectronic properties of two-dimensional materials.

[0003] The van der Waals heterostructures formed by vertically stacking 2D TMDs not only inherit the original properties of individual materials but also exhibit many new phenomena, such as interlayer excitons (IX). Interlayer excitons exhibit various excellent optoelectronic properties, such as out-of-plane dipole moments, long exciton lifetimes, and valley polarization lifetimes. Interlayer excitons can be trapped by the defect potentials generated by defect structures, resulting in defect-related optical behaviors, further expanding their optoelectronic applications. The defect potential can reduce the PL emission and lifetime of IX by capturing carriers and increasing the non-radiative rate, which has been reported in WS2 / WSe2 heterobilayers. In nearly aligned (0°) or anti-aligned (60°) heterobilayers, interlayer excitons can be confined within the periodic moiré potential, forming the so-called moiré interlayer excitons (moiré-IX). Moiré interlayer excitons exhibit alternating circular polarization and multi-peak emission, with an extremely narrow linewidth and single-photon emission characteristics. Theoretically, the moiré potential and the defect potential can compete and cooperate with each other to jointly regulate the optical properties of IX. Although the observation of local defect states and moiré potential in the MoSe2 / WSe2 heterobilayer has been reported, resulting in significant differences in the emission behavior of IX at different temperatures. However, the modulation of the moiré potential and defect potential on interlayer exciton localization and valley polarization dynamics in TMDs heterobilayers has not been explored, which hinders the large-scale application of two-dimensional optoelectronic devices. Therefore, controlling the modulation of defects and moiré potential on interlayer excitons is extremely important. Summary of the Invention

[0004] Aiming at the problems existing in the prior art, the first object of the present invention is to provide a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization. Based on the synergistic effect of a large number of chalcogen atom vacancy defects in the monolayer two-dimensional transition metal chalcogenide alloy and the twist angle between the monolayer two-dimensional transition metal chalcogenide, the optical emission behavior of the interlayer exciton is jointly regulated by the chalcogen atom vacancy defects and the moiré periodic potential of the heterolayer, thereby realizing the tunable control of the interlayer exciton localization of the two.

[0005] The second object of the present invention is to provide a preparation method of a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization. By stacking according to a specific twist angle to generate a double-well competition regulation mechanism, the tunable interlayer exciton localization is realized. Among them, through the analysis of the temperature-dependent PL spectra and lifetimes, the activation energies of the defect interlayer excitons and moiré interlayer excitons of the heterojunction material provided by the present invention are 20 meV and 22.2 meV respectively. Therefore, the interlayer excitons in the heterojunction at an interlayer twist angle of 0 to 3° are trapped by the moiré potential, while at an interlayer twist angle of 3 to 22°, the interlayer excitons are trapped by the defect potential.

[0006] To achieve the above technical objectives, the present invention provides a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization. The material consists of a two-dimensional heterojunction material and a hard substrate; the two-dimensional heterojunction material includes a monolayer two-dimensional transition metal chalcogenide and a monolayer two-dimensional transition metal chalcogenide alloy; there is no slip between the substrate and the two-dimensional heterojunction material;

[0007] The interlayer twist angle between the monolayer two-dimensional transition metal chalcogenide and the monolayer two-dimensional transition metal chalcogenide alloy is 0 to 3° or 3 to 22°.

[0008] In the technical solution provided by the present invention, first, a heterojunction structure is constructed by "transition metal chalcogenide alloy - transition metal chalcogenide pure phase". By using the chalcogen vacancy defects rich in the sulfide alloy, a controllable defect potential well is introduced. Then, by controlling the twist angle between the two, two twist angles of small-angle quasi-coherent and large-angle quasi-incoherent are set to realize the tunable control of the interlayer exciton localization of the two. In addition, since the moiré IX exhibits cross-circularly polarized emission due to strong spatial confinement, and the defect IX is enhanced due to phonon coupling, therefore, by controlling the temperature of the heterojunction, the specific valley polarization state can be selectively enhanced to realize the tunable valley polarization.

[0009] As a preferred solution, the chemical general formula of the two-dimensional transition metal chalcogenide is MX2, where M is a transition metal element and X is S or Se.

[0010] As a preferred solution, the chemical general formula of the two-dimensional transition metal chalcogenide alloy is MX2-2x X' 2x , where M is a transition metal element, X is S or Se, X' is S or Se, and X and X' are different from each other.

[0011] As a preferred solution, the transition metal element is one of Ti, In, Ta, Mo, W, Re, and Te. Further preferably, the transition metal element is W.

[0012] As a preferred solution, the fluorescence spectrum of the monolayer two-dimensional transition metal chalcogenide at 10 - 300 K does not overlap with that of the monolayer two-dimensional transition metal chalcogenide alloy. The selection of the transition metal chalcogenide and the transition metal chalcogenide alloy should be strictly carried out according to the above requirements. The non-overlap of their fluorescence spectra in this temperature range can effectively avoid the spectral interference between intra-layer excitons and inter-layer excitons. For example, the A exciton (~2.0 eV) of WS2 and the defect state (~1.6 eV) of WS 0.9 Se 1.1 form a clear energy band offset, which is beneficial to the selective excitation and detection of inter-layer excitons.

[0013] As a preferred solution, the hard substrate is a silicon-based substrate with the function of shielding the interference of the external dielectric environment. Further preferably, the hard substrate is silicon and / or silicon dioxide.

[0014] As a preferred solution, when the inter-layer twist angle between the monolayer two-dimensional transition metal chalcogenide and the monolayer two-dimensional transition metal chalcogenide alloy is 0 - 3°, the inter-layer excitons fall into the moiré potential trap.

[0015] As a preferred solution, when the inter-layer twist angle between the monolayer two-dimensional transition metal chalcogenide and the monolayer two-dimensional transition metal chalcogenide alloy is 3 - 22°, the inter-layer excitons fall into the defect potential trap.

[0016] The heterojunction material provided by the present invention first realizes the tunability of inter-layer exciton localization by adopting a double potential well competition regulation mechanism. When the twist angle is at a small angle, the periodic moiré potential forms a deep potential well, and the exciton is localized as moiré IX. When the twist angle is at a large angle, the moiré potential period increases, resulting in a shallower potential well, and the defect potential becomes dominant, and the exciton is localized as defect IX. Therefore, by adjusting the twist angle, the reversible switching of the exciton capture mode can be realized.

[0017] The present invention also discloses a preparation method of a two-dimensional heterojunction material with tunable inter-layer exciton localization and valley polarization. The monolayer two-dimensional transition metal chalcogenide and the monolayer two-dimensional transition metal chalcogenide alloy are respectively obtained by mechanical exfoliation method, and the two are stacked on the hard substrate in sequence according to the twist angle, and annealed layer by layer to obtain the product.

[0018] As a preferred solution, the conditions for the annealing treatment are as follows: the vacuum degree is 0.8×10 -4 mbar ~ 4.2×10 -5 mbar, the temperature is 130 - 180 °C, and the time is 6 - 8 h.

[0019] Compared with the prior art, the beneficial technical effects of the technical solution of the present invention are as follows:

[0020] 1) The heterojunction material provided by the present invention is based on the synergistic effect of a large number of chalcogen atom vacancy defects in the monolayer two-dimensional transition metal chalcogenide alloy and the twist angle with the monolayer two-dimensional transition metal chalcogenide. By using the chalcogen atom vacancy defects and the moiré periodic potential of the heterolayer, the optical emission behavior of the interlayer excitons is jointly regulated, thereby realizing the tunable control of the localization of the interlayer excitons of the two; the dual-mode excitons of defect IX and moiré IX provided in the present invention can be manipulated in parallel through wavelength selection, which can significantly improve the integration of optoelectronic devices.

[0021] 2) In the preparation method provided by the present invention, a double-well competition regulation mechanism is generated by stacking according to a specific twist angle, and then the tunable localization of the interlayer excitons is realized. Among them, through the analysis of the temperature-dependent PL spectrum and lifetime, the activation energies of the defect interlayer excitons and moiré interlayer excitons of the heterojunction material provided by the present invention are 20 meV and 22.2 meV respectively. Therefore, the interlayer excitons in the heterojunction at an interlayer twist angle of 0 - 3° are trapped by the moiré potential, while at an interlayer twist angle of 3 - 22°, the interlayer excitons are trapped by the defect potential.

[0022] 3) In the technical solution provided by the present invention, based on the phonon-assisted valley depolarization kinetic characteristics of the two types of interlayer excitons, through the circularly polarized time-resolved PL spectrum, the valley kinetic differences between the defect interlayer excitons with co-circularly polarized PL emission behavior and the moiré interlayer excitons with cross-circularly polarized PL emission are analyzed. Through temperature regulation, a specific valley polarization state can be selectively enhanced. In addition, since the defect IX reaches a valley polarization rate of 24.5% at 14 K under non-resonant excitation, which is greater than that of the "pure-phase - pure-phase TMD heterojunction" system in the prior art (the polarization rate is about 10% at 10 K). Description of the Drawings

[0023] Figure 1 It is a schematic flow chart for the preparation of monolayer materials and heterobilayer materials in the examples and comparative examples; the samples are annealed layer by layer.

[0024] Figure 2 It is a schematic diagram of the atomic structure and a morphological characterization diagram of the heterobilayer obtained in Example 1;

[0025] Among them, Figure 2 (a) is the heterobilayer WS2 / WS0.9 Se 1.1 Schematic diagram of atomic structure and spatial distributions of Defect-X, Defect-IX, and Moiré-IX excitons in alloy monolayer WS 0.9 Se 1.1 Chalcogen atom vacancy defects in are marked with arrows; Figure 2 (b) shows WS 0.9 Se 1.1 Alloy and WS2 / WS 0.9 Se 1.1 The simplified energy band structure of the heterostructure shows Defect-X, Defect-IX, and Moiré-IX transitions. Electrons in the defect energy level and holes in the valence band form Defect-X. Defect-IX is formed by the transition of electrons in the defect energy level and holes at strongly orbital hybridized Γ. Electrons at the K point in the WS2 layer and holes at strongly orbital hybridized Γ combine to form Moiré-IX; Figure 2 (c) shows the bright-field optical image of the WS2 / WS 0.9 Se 1.1 heterobilayer on the SiO2 / Si substrate. The inset shows the polarization-resolved SHG signals of the top WS 0.9 Se 1.1 and bottom WS2 monolayer regions, determining that the twist angle between the heterobilayer regions (HB) is approximately 6.4°; Figure 2 (d) shows the atomic-resolution HAADF-STEM image of HB. The basic repeating unit of the Moiré superlattice is approximately 3 nm and is marked with a parallelogram. The inset is an enlarged view of the lattice of monolayer WS 0.9 Se 1.1 alloy. The dashed arrow line indicates the detected chalcogen atom vacancy defect;

[0026] Figure 3 It is a more detailed atomic structure and morphological characterization diagram of the monolayer two-dimensional TMDs alloy in the heterobilayer obtained in Example 1;

[0027] Among them, Figure 3 (a) and Figure 3 (b) are the atomic-resolution HAADF-STEM images of WS2 / WS 0.9 Se 1.1 heterobilayer and monolayer WS 0.9 Se 1.1 respectively; In Figure 3 (a), the Moiré superlattice is marked with a parallelogram. The interface between monolayer WS 0.9 Se 1.1 alloy and the heterobilayer HB is marked with a line. The inset is the FFT pattern of WS2 / WS 0.9 Se 1.1 heterobilayer; In Figure 3 (b), monolayer WS 0.9 Se1.1 Typical chalcogen atom vacancy defects in the alloy are marked with arrows; the inset is a monolayer WS 0.9 Se 1.1 FFT pattern of the alloy, where d is the lattice constant; Figure 3 (c) is a monolayer WS 0.9 Se 1.1 EDS spectrum of the alloy;

[0028] Figure 4 For the monolayer WS in Comparative Example 3 0.9 Se 1.1 Elemental characterization diagram of the alloy;

[0029] Among them, Figure 4 (a) is a monolayer WS 0.9 Se 1.1 SEM surface scanning image of the alloy; Figure 4 (b)~ Figure 4 (d) are monolayer WS 0.9 Se 1.1 The quantitative ratio of the elemental mappings of W, Se, and S in the alloy is 2724:2998:2209.

[0030] Figure 5 For the WS2 / WS with different interlayer twist angles provided in Example 1 and Example 2 0.9 Se 1.1 Heterobilayers and the corresponding monolayer WS2 and WS 0.9 Se 1.1 Room temperature Raman spectra;

[0031] Figure 6 For the WS2 / WS with different combinations of interlayer twist angles provided in Example 1 and Example 2 0.9 Se 1.1 Comparison of the PL spectra and lifetimes of different types of excitons in the heterobilayers;

[0032] Among them, Figure 6 (a) Under 488 nm continuous wavelength (CW) laser excitation, the normalized PL spectra comparisons of HB (interlayer twist angles: 1.4° and 6.4°), monolayer WS 0.9 Se 1.1 and WS2 regions at 10 K (red curve) and 295 K (black dashed line) respectively, X0: neutral exciton; Figure 6 (b) The normalized time-resolved PL spectra (TRPL) of the corresponding Moiré-IX, defect-IX, defect-X, WS 0.9 Se 1.1 -X and WS2-X at 14 K under 640 nm pulsed laser excitation;

[0033] Figure 7 For the single-layer WS in Comparative Example 3 0.9 Se 1.1 Power-dependent PL spectra of defect-X excitons in the alloy;

[0034] Among them, Figure 7 (a) is the power-dependent PL spectrum of the single-layer WS 0.9 Se 1.1 alloy; Figure 7 (b) and Figure 7 (c) are the relationships between the exciton peak energies and PL intensities of X0 and defect-X extracted from Figure 7 (a) and the excitation power.

[0035] Figure 8 For the power-dependent PL spectra of defect-IX excitons in the WS2 / WS 0.9 Se 1.1 heterobilayer provided in Example 1;

[0036] Among them, Figure 8 (a) is the comparison of PL spectra of defect-IX exciton emission in the HB region of Example 1 at excitation powers of 0.1 μW and 40 μW; Figure 8 (b) is the power-dependent PL spectrum of defect-IX exciton emission; Figure 8 (c) is the relationship between the excitation power and the exciton PL intensity and peak position of defect-IX extracted and fitted from Figure 8 (b).

[0037] Figure 9 For the power-dependent PL spectra of Moiré-IX excitons in the heterobilayer provided in Example 2;

[0038] Among them, Figure 9 (a) is the bright-field optical images of the single-layer WS in the comparative example 0.9 Se 1.1 alloy and WS2; Figure 9 (b) is the polarization-dependent SHG signal of the top single-layer WS in the comparative example 0.9 Se 1.1 alloy and the bottom single-layer WS2 region, determining that the twist angle between the HB regions is approximately 1.4°; Figure 9 (c) is the power-dependent PL spectrum of the WS2 / WS 0.9 Se 1.1 heterobilayer; Figure 9 (d) is the relationship between the PL intensity and the excitation power extracted from Figure 9 (c).

[0039] Figure 10Steady-state PL spectra of different exciton types in Example 1 and Example 2 as a function of temperature;

[0040] Among them, Figure 10 (a)~ Figure 10 (c) are temperature-dependent PL spectra of defect-X, defect-IX, and moiré-IX exciton emissions under an excitation power of 3 μW; Figure 10 (d)~ Figure 10 (f) are Figure 10 (a)~ Figure 10 (c) temperature-dependent normalized PL spectra; Figure 10 (g)~ Figure 10 (i) are Figure 10 (a)~ Figure 10 (c) the variations of exciton PL intensities of defect-X, X0, defect-IX, and moiré-IX extracted from

[0041] Figure 11 Temperature-dependent circularly polarized PL spectra and corresponding circular polarization degrees (ρ) of different interlayer excitons in Example 1 and Example 2;

[0042] Among them, Figure 11 (a) and Figure 11 (c) are circularly polarized PL spectra of the HB region (twist angles are 6.4° and 1.4° respectively) in the temperature range of 14~50 K under σ + excitation (640 nm); Figure 11 (b) and Figure 11 (d) are the variations of circular polarization degree ρ with energy in the temperature range of 14~50 K calculated according to Figure 11 (a) and Figure 11 (c) respectively; Figure 11 (e) is the variation of exciton valley polarization degree ρ of defect-IX, moiré-IX, and defect-X calculated, and the valley polarization degree data used are taken from Figure 11 (b) and Figure 11 (d) the arrows in; Figure 11 (f) is a schematic diagram of the competition of interlayer excitons captured by the moiré potential and the defect potential.

[0043] Figure 12 Temperature-dependent circular polarization-resolved PL spectra of defect-X excitons in monolayer WS 0.9 Se 1.1 alloy in Comparative Example 3;

[0044] Among them, Figure 12 (a) is the evolution of the circularly polarized PL spectrum of defect-X in monolayer WS 0.9 Se 1.1 alloy; Figure 12 (b) is Figure 12Relationship between valley polarization degree and energy corresponding to (a).

[0045] Figure 13 For WS2 / WS with different interlayer twist angles provided in Example 1 and Example 2 0.9 Se 1.1 Circular polarization-resolved TRPL spectra of interlayer excitons as a function of temperature in the heterobilayer;

[0046] Among them, Figure 13 (a) and Figure 13 (b) are the exciton-normalized circular polarization time-resolved PL spectra of defect IX and moiré-IX respectively. All spectra are excited by a 640 nm laser with a power of 15 μW; Figure 13 (c) and Figure 13 (d) are the dynamic evolutions of valley polarization as a function of time for defect-IX and moiré-IX at different temperatures respectively corresponding to Figure 13 (a) and Figure 13 (b). They increase the inter-valley depolarization time with the decrease of temperature. The curves are double-exponential fitting lines, and the lifetime values obtained from all fittings are the average of the two recombination lifetime components; Figure 13 (e) and Figure 13 (f) are the mechanisms of the depolarization processes of defect-IX and moiré-IX respectively. The double-arrow straight lines respectively represent the recombination processes with σ － (σ ＋ ) polarization. The width represents the number of radiated photons, and the solid-arrow of the curve represents the carrier valley-to-valley relaxation process, and the thickness represents the amount of carrier relaxation.

[0047] Figure 14 Circular polarization-resolved TRPL spectra of defect-X excitons as a function of temperature in the single-layer alloy in Comparative Example 3;

[0048] Among them, Figure 14 (a)~ Figure 14 (c) are the normalized circular polarization time-resolved PL spectra of defect-X excitons in the single-layer WS 0.9 Se 1.1 alloy at temperatures of 14 K, 30 K and 50 K;

[0049] Figure 15 PL spectra of defect-IX excitons as a function of power in the heterobilayer provided in Comparative Example 1;

[0050] Among them, Figure 15 (a) is the bright-field optical image of the single-layer WS 0.9 Se 1.1 alloy and WS2 in the comparative example; Figure 15 (b) is the top single-layer WS in the comparative example 0.9 Se1.1 The polarization-dependent SHG signals of the alloy and the bottom single-layer WS2 region were used to determine that the twist angle between the HB regions was approximately 22.3°; Figure 15 (c) is for WS2 / WS 0.9 Se 1.1 Power-dependent PL spectra of the heterobilayer; Figure 15 (d) is Figure 15 The relationship between the PL intensity extracted from (c) and the excitation power. Detailed implementation manners

[0051] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below with reference to specific implementation cases. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, these embodiments are provided to make the understanding of the disclosure of the present invention more thorough and comprehensive. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0052] Example 1

[0053] This example provides a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization, and its preparation process is as follows:

[0054] 1) Using tape, thin-layer materials were exfoliated from the bulk WS2, WS 0.9 Se 1.1 source, and then the tape was folded and torn in half three times to dissociate thinner materials. A PDMS film with a size of about 1×1 cm was attached to one side surface of the glass slide, and then the tape with the material was attached to the PDMS film; the tape was gently pressed to ensure that the material on the surface was in uniform and close contact with the PDMS. After waiting for 3-5 minutes, the tape was peeled off from one side of the PDMS, and a part of the dissociated sample remained on the PDMS surface; the surrounding part of the PDMS containing the target sample in the single-layer region was cut off with a knife;

[0055] 2) Under a microscope, the target sample on the PDMS surface was transferred to a 300 nm SiO2 / Si substrate through a three-dimensional transfer platform, and then another single-layer sample was stacked up and down to form WS2 / WS 0.9 Se 1.1Heterogeneous bilayer region. To ensure an interlayer twist angle of 6 - 7°, with reference to the included angle between the long sides of the upper and lower single-layer samples, the angles of the microscope and the rotation transfer platform are combined to precisely control the parallelism of the long sides of the upper and lower single-layer samples. Finally, the prepared target heterogeneous bilayer sample is placed in a vacuum annealing furnace for annealing to reduce interlayer bubbles and impurities and enhance interlayer coupling. The vacuum degree in the furnace is maintained at 3.0×10 -5 mbar, and the condition of a temperature of 130 °C is maintained for 6 hours to obtain the heterojunction material.

[0056] Example 2

[0057] The preparation method of this comparative example is exactly the same as that of Example 1, except that: the interlayer twist angle of the prepared heterogeneous bilayer is 1.4°.

[0058] Comparative Example 1

[0059] The preparation method of this comparative example is exactly the same as that of Example 1, except that: the interlayer twist angle of the prepared heterogeneous bilayer is 22 - 23°.

[0060] Comparative Example 2

[0061] The preparation method of this comparative example is exactly the same as that of Example 1, except that: the prepared heterogeneous bilayer material is not subjected to the annealing operation.

[0062] Comparative Example 3

[0063] The preparation method of this comparative example is exactly the same as that of Example 1, except that: the prepared material is only the precursor single-layer WS 0.9 Se 1.1 alloy.

[0064] To better illustrate the characteristics and advantages of different excitons in the samples prepared by the present invention, the present invention also carried out a series of characterizations and tests on the materials obtained from the above examples and different comparative examples:

[0065] The WS2 / WS 0.9 Se 1.1 heterogeneous bilayer obtained from Example 1 and Example 2 is schematically shown in its typical atomic structure diagram in Figure 2 (a), where the single-layer WS 0.9 Se 1.1 alloy containing a large number of chalcogen atom vacancy defects is stacked on the single-layer WS2. Due to the presence of vacancy defects, electrons and holes in the single-layer WS 0.9 Se 1.1 alloy form defect-bound intra-layer excitons (defect-X) under photoexcitation. When the single-layer WS 0.9 Se 1.1When the electrons in the alloy combine with the holes in WS2, defect interlayer excitons (defect-IX) can be formed. Such interlayer excitons can also be trapped by the moiré potential to form moiré interlayer excitons (moiré-IX). To illustrate the distributions of defect-X, defect-IX, and moiré-IX in the heterobilayer band alignment, we refer to the previous research results and draw a simplified schematic diagram of the band alignment ( Figure 2 Figure (b)). The valence band holes on the defect energy levels of the single-layer alloy combine with electrons to form defect-X. The defect energy level electrons in the WS 0.9 Se 1.1 alloy layer and the holes at the heterobilayer hybridized Γ point form defect-IX with K-Γ transition, while the K valley electrons in the WS2 layer and the holes at the heterobilayer hybridized Γ point form moiré-IX. We fabricated WS2 / WS 0.9 Se 1.1 heterobilayers on the SiO2 / Si substrate using the PDMS-assisted site-specific transfer technique and performed vacuum annealing treatment on them to ensure clean contact between the interfaces ( Figure 2 as shown in Figure (c)). The composition of the single-layer WS 0.9 Se 1.1 alloy was determined by energy-dispersive X-ray spectroscopy and scanning electron microscopy elemental mapping, as shown in Figure 3 and Figure 4 . The Raman vibration modes in the heterobilayers of Example 1 and Example 2 both showed a wavenumber redshift compared with those in the single layer, indirectly indicating strong interlayer coupling, as shown in Figure 5 . The stacking angle of the heterobilayer was determined by polarization-resolved SHG, as shown in the inset of Figure 2 Figure (c). To characterize the atomic structure of the twisted WS2 / WS 0.9 Se 1.1 heterobilayer of Example 1 and the defects in the single-layer WS 0.9 Se 1.1 alloy, we transferred the samples onto copper grids for high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements. Figure 2 Figure (d) shows the moiré superlattice of the WS2 / WS 0.9 Se 1.1 heterobilayer with an interlayer twist angle of approximately 6.4°, and its superlattice constant is approximately 3.0 nm ( Figure 3 schematically shows more detailed atomic structure features such as lattice constant and interlayer twist angle, etc.), and the uncertainty of its theoretical value compared with the experimental data is approximately 2.5%. More detailed STEM images of the single-layer alloy WS 0.9 Se 1.1 show a clearly distinguishable hexagonal lattice of chalcogen atoms and metal atoms, which contains many chalcogen element atom vacancy defects, as shown in Figure 3 Figure (b) andFigure 2 as shown in the inset of (d).

[0066] This invention mainly studied the influence of chalcogen atom vacancies in the composites of Example 1 and Example 2 on the exciton states in the twisted TMDs heterobilayer. Steady-state PL spectroscopy analysis was carried out under continuous light excitation with a wavelength of 488 nm, as Figure 6 shown in (a). By measuring the PL spectra at 295 K, we observed a low-energy interlayer exciton emission peak in the heterobilayers with twist angles of 1.4° and 6.4°, and an in-plane exciton X peak that does not exist in monolayer WS 0.9 S e1.1 alloy and WS2. At 10 K, the in-plane exciton X complex in the monolayer of the heterobilayer was almost quenched, and the intensity of IX dominated. Three different IXs appeared in the heterobilayer obtained in Example 2, with an energy spacing of about 60 meV, which is consistent with the characteristics of moiré interlayer excitons. The IX in the heterobilayer in Example 1 had the same energy as the low-energy peak observed in the monolayer WS 0.9 Se 1.1 alloy in Comparative Example 3. We inferred that there was a close correlation between the two. Through the linear fitting coefficient of power-PL intensity (α = 0.24, ), as Figure 7 shown, it was proved that the strong low-energy broad peak with a full width at half maximum (FWHM) of up to 150 meV in the monolayer WS 0.9 Se 1.1 alloy in Comparative Example 3 originated from chalcogen atom vacancy defects, which was consistent with the existing literature. Due to the dipole repulsion between excitons, the PL spectrum of the interlayer exciton showed a significant blue shift with the increase of power. The power-dependent PL intensity fitting in Example 1 showed that the intensity increased sublinearly with power (α = 0.85), further proving that it might be trapped by the defect potential ( Figure 8 shown). In contrast, the moiré potential-confined interlayer exciton in Example 2 had only 0.78, indicating that the interlayer exciton was trapped by a deeper moiré potential and was more difficult to delocalize from the potential ( Figure 9 shown).

[0067] Next, this invention carried out temperature-dependent PL spectroscopy analysis on the examples and heterojunction examples to further explore the possibility that defect-IX might be related to the defect-X emission characteristics in monolayer WS 0.9 Se 1.1 alloy. Figure 10 (a)–(c) respectively show the WS 0.9 Se 1.1 alloy monolayer and WS2 / WS 0.9 Se 1.1Comparison of the PL spectra of interlayer excitons in the heterostructure. As the temperature increases, their emission quenching processes and energy redshift trends are very similar, which is consistent with the temperature-dependent behavior of the semiconductor bandgap. Due to the weakening of phonon effects at low temperatures, the rate of interlayer charge transfer is increased, resulting in much greater PL intensities of defect-IX ( Figure 10 as shown in (b)) and moiré-IX ( Figure 10 as shown in (c)) than that of defect-X ( Figure 10 as shown in (a)). As shown in the temperature-dependent PL spectra diagrams with intensity normalization ( Figure 10 as shown in (d), 10(e)), the PL intensities of defect-X and defect-IX are completely quenched at about 80 K, which may imply the existence of similar defects. The PL intensity of moiré-IX is quenched near 200 K and changes from multiple peaks to a single peak as the temperature increases ( Figure 10 as shown in (c), 10(f)), indicating that the interlayer excitons are indeed trapped by the moiré potential.

[0068] Figure 10 (g)–(i) respectively show the variation of the PL intensities of different excitons extracted from Figure 10 (a)–(c) with temperature. We use the Arrhenius equation to analyze their radiative and non-radiative processes with temperature change, where is the Boltzmann constant, is the activation energy, and A is proportional to the thermal dissociation rate. After averaging through multiple fitting calculations, the activation energies of defect-X and defect-IX are almost the same, about 20 meV ( Figure 10 as shown in (g), 10(h)), indicating that their non-radiative dissociation electron transition channels may be the same, which further provides one of the evidences that the interlayer excitons are confined by chalcogen atom vacancies. The activation energy (103 meV) of the neutral exciton in the alloy monolayer WS 0.9 Se 1.1 is much greater than that of defect-X, indicating that defect-X is impossible to appear at room temperature ( Figure 10 as shown in (g)). Figure 10 (i) shows that the activation energy of moiré-IX is 22.3 meV, which is greater than that of defect-IX.

[0069] The present invention further studies the valley polarization kinetic processes of the interlayer excitons trapped by defects and the moiré potential in Example 1 and Example 2 at different temperatures. The valley polarization degree is related to the exciton lifetime ( ) and the valley depolarization time ( ) and can be understood by the formula , where is the polarization degree without valley relaxation. Figure 13(a) and 13(b) respectively show the circularly polarized-resolved PL decays of moiré-IX and defect-IX under 640 nm pulsed laser excitation. As the temperature increases from 14 K to 50 K, the difference between the and components decreases, which is consistent with the results in Figure 11 (a), 11(c). When the temperature increases from 14 K to 50 K, the and component lifetimes of defect-IX decrease from 34.5 ns and 20.5 ns to 8.5 ns and 8.4 ns respectively. For moiré-IX, these lifetimes decrease from 25.8 ns and 13.1 ns to 7.9 ns and 6.4 ns. Similarly, for the monolayer WS 0.9 Se 1.1 alloy in Comparative Example 3, the circular polarization degree of defect-X is close to 0 ( Figure 12 ), and its and component lifetimes both decrease from about 56 ns at 14 K to about 22 ns at 50 K, as shown in Figure 14 . Under the same experimental conditions, the exciton lifetime of defect-IX is still longer than that of moiré-IX. This may be due to the extended exciton lifetime caused by the larger momentum space mismatch of defect excitons.

[0070] To better understand the generation mechanism of valley polarization in the present invention, we calculated the change of valley polarization degree with time according to the lifetimes of the and components, defined as , where and are the PL intensities of the and components at time t respectively. Figure 13 (c), 13(d) summarize the change of valley polarization degree with time at different temperatures. When the temperature increases from 14 K to 50 K, the initial valley polarization degrees of defect-IX and moiré-IX at time 0 decrease from 0.24 to 0 and from 0.35 to 0.15 respectively. When the temperature increases from 14 K to 40 K, the valley depolarization lifetime of defect-IX decreases from 10.5 ns to 2.2 ns. Compared with defect-IX, moiré-IX has a shorter exciton lifetime and a longer valley depolarization lifetime ( decreases from 52 ns at 14 K to about 2.3 ns at 50 K), resulting in a larger valley polarization degree ρ. The obtained and The valley polarization of defect-IX is calculated to decrease from about 0.33 at 14 K to 0.17 at 40 K, which is slightly smaller than the observed circular polarization intensity. Considering that the 640 nm excitation light is not resonant with the energy of defect-IX and moiré-IX, The value of is small, so the obtained and The observed valley polarization The valley polarizability of defect-IX is smaller than that of moiré-IX at the same temperature. This is because defect-IX has a stronger phonon thermal activation effect, which leads to a faster phonon-assisted valley scattering rate. Under light excitation, a large amount of The emitted defect -IX, some of which undergo spin flipping under the action of phonons and relax to the -K valley ( Figure 13 (e) Compared with defect-IX, the circular polarization degree of moiré-IX in Example 2 is higher due to its recombination time Faster relaxation time slower, resulting in fewer excitons relaxing to the -K valley ( Figure 13 (f)).

[0071] In summary, the heterojunction material provided by the present invention can tune the interlayer excitons to be localized by periodic moiré potential or defect potential by changing the twist angle. The valley polarization of the cross-circularly polarized moiré interlayer excitons and the co-circularly polarized defect interlayer excitons can be tuned by temperature.

[0072] The present invention also compares the heterogeneous double layer in Example 1 with the twist angle of 22.3°. After testing, it is found that its low-temperature PL spectrum shows that the linear fitting coefficient of the power dependence of its interlayer exciton on the PL intensity is 0.34, as shown in FIG. Figure 15 When the angle range specified by the present invention is exceeded, the tuning mechanism of interlayer exciton localization fails, the state of the interlayer excitons will no longer show the previous trend, and their behavior is uncontrollable.

[0073] The present invention compares Example 1 and Comparative Example 3, and tests the low-temperature spectrum of the heterogeneous double layer before annealing. It is found that the fluorescence intensity of the intralayer excitons of the heterogeneous double layer in Comparative Example 2 is higher than the intensity of the interlayer excitons, indicating that its interlayer coupling is poor. After further annealing, it becomes a heterogeneous double layer with strong interlayer coupling in Examples 1 and 2.

[0074] The embodiments of the present invention and all comparative examples have been described above in conjunction with the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative rather than restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the spirit of the present invention and the scope protected by the claims. All of these fall within the protection scope of the present invention.

Claims

1. A two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization, characterized in that: It is composed of a two-dimensional heterojunction material and a hard substrate; the two-dimensional heterojunction material includes a single layer of two-dimensional transition metal chalcogenide and a single layer of two-dimensional transition metal chalcogenide alloy; there is no slip between the hard substrate and the two-dimensional heterojunction material; The interlayer torsion angle between the single-layer two-dimensional transition metal chalcogenide and the single-layer two-dimensional transition metal chalcogenide alloy is 0-3° or 3-22°; The chemical formula of the two-dimensional transition metal chalcogenide is MX2, wherein M is a transition metal element and X is S or Se; The chemical formula of the two-dimensional transition metal chalcogenide alloy is MX 2-2x X' 2x , wherein M is a transition metal element, X is S or Se, X' is S or Se, and X and X' are different; The transition metal element is one of Ti, In, Ta, Mo, W, Re and Te; the fluorescence photoinduced spectrum of the single-layer two-dimensional transition metal chalcogenide at 10-300K does not overlap with that of the single-layer two-dimensional transition metal chalcogenide alloy.

2. The two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization according to claim 1, characterized in that: The hard substrate is a silicon-based substrate having the function of shielding interference from the external dielectric environment.

3. The two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization according to claim 1, characterized in that: When the interlayer twist angle between the single-layer two-dimensional transition metal chalcogenide and the single-layer two-dimensional transition metal chalcogenide alloy is 0-3°, the interlayer excitons fall into the moiré potential trap.

4. The two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization according to claim 1, characterized in that: When the interlayer twist angle between the single-layer two-dimensional transition metal chalcogenide and the single-layer two-dimensional transition metal chalcogenide alloy is 3-22°, the interlayer excitons fall into a defect potential trap.

5. The method for preparing a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization according to any one of claims 1 to 4, characterized in that: A single-layer two-dimensional transition metal sulfide and a single-layer two-dimensional transition metal sulfide alloy are obtained by mechanical exfoliation, and the two are stacked on a hard substrate in sequence according to a torsion angle, and annealed layer by layer to obtain the result.

6. The method for preparing a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization according to claim 5, characterized in that: The annealing conditions are: vacuum degree of 0.8×10 -4 mbar ~4.2×10 -5 mbar, temperature is 130~180°C, and time is 6~8h.

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