Interlayer exciton localization and valley polarization tunable two-dimensional heterojunction material and preparation method thereof

Abstract

The invention discloses an interlayer exciton localization and valley polarization tunable two-dimensional heterojunction material and a preparation method thereof. The material is composed of a two-dimensional heterojunction material and a hard substrate. The two-dimensional heterojunction material comprises a single-layer two-dimensional transition metal chalcogenide and a single-layer two-dimensional transition metal chalcogenide alloy; no slippage exists between the substrate and the two-dimensional heterojunction material; and the interlayer torsion angle between the single-layer two-dimensional transition metal chalcogenide and the single-layer two-dimensional transition metal chalcogenide alloy is 0.1-3 degrees or 3-22 degrees. The material is obtained by stacking single-layer raw materials on a hard substrate by adopting a mechanical stripping method and carrying out layer-by-layer annealing treatment. According to the material, the technical effects of localization and valley polarization of different interlayer excitons are achieved by controlling the interlayer torsion angle of the heterogeneous double layers by utilizing a double-potential-well competition regulation mechanism of a chalcogenide atom vacancy defect between the materials and Moire period potential, and the material can be used for design optimization of spin-valley photoelectric equipment of defect engineering.

Description

Technical Field

[0001] The present invention relates to a two-dimensional heterojunction material, in particular 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 2D TMDs monolayers play an important role in their electronic and optoelectronic properties. Various defect structures are common in 2D TMDs synthesized using existing synthesis techniques and manufacturing tools. Especially in alloys, the most common defects in 2D TMDs are chalcogenide atom vacancy defects because they are easily lost during growth or post-processing. In terms of electronic properties, in-situ replacement 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, even in the initially stoichiometric 2D PtSe 2 The semiconductor-to-metal transition is achieved in the layer. In terms of optoelectronic properties, chalcogen vacancy defects usually lead to defect-related energy states, resulting in the formation of defect-bound excitons, the extension of exciton lifetime, and a wide range of photoluminescence (PL) energy emission (half-wave width > 100 meV). Therefore, understanding and controlling defects is of great significance for exploring and regulating the optoelectronic properties of two-dimensional materials.

[0003] The van der Waals heterostructure formed by vertically stacking 2D TMDs not only inherits the original properties of the individual materials, but also produces many new phenomena, such as interlayer excitons (IX). Interlayer excitons exhibit various excellent optoelectronic properties, such as out-of-plane dipole moment, long exciton lifetime, and valley polarization lifetime. Interlayer excitons can be bound by the defect potential generated by the defect structure, resulting in defect-related optical behaviors, further expanding their optoelectronic applications. The defect potential can reduce the PL emission and lifetime of IX by trapping carriers and increasing the non-radiative rate, which is shown in WS. 2 / WSe 2 It has been reported in heterobilayers. In nearly aligned (0°) or anti-aligned (60°) heterobilayers, interlayer excitons can be confined within a periodic moiré potential to form so-called moiré interlayer excitons (Moiré-IX). Moiré interlayer excitons exhibit alternating circular polarization and multi-peak emission with extremely narrow linewidth and single-photon emission characteristics. In theory, the moiré potential and defect potential can compete and cooperate with each other to jointly regulate the optical properties of IX. Although MoSe has been reported 2 / WSe 2The observation of localized defect states and moiré potential in heterobilayers leads to significant differences in the emission behavior of IX at different temperatures. However, the modulation of interlayer exciton localization and valley polarization dynamics by moiré and defect potentials in TMDs heterobilayers remains unexplored, which hinders the large-scale application of 2D optoelectronic devices. Therefore, it is extremely important to control the modulation of interlayer excitons by defects and moiré potentials. Summary of the invention

[0004] In view of 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 chalcogenide atom vacancy defects in a single-layer two-dimensional transition metal chalcogenide alloy and the torsion angle between the single-layer two-dimensional transition metal chalcogenide, the optical emission behavior of the interlayer excitons is jointly regulated by the chalcogenide atom vacancy defects and the moiré periodic potential of the heterolayer, thereby achieving tunable control of the interlayer exciton localization of the two.

[0005] The second object of the present invention is to provide a method for preparing a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization, by generating a double potential well competition regulation mechanism by stacking according to a specific twist angle, thereby achieving tunable interlayer exciton localization, wherein, through analysis of temperature-dependent PL spectra and lifetimes, the activation energies of 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 captured by the moiré potential, while at an interlayer twist angle of 3~22°, the interlayer excitons are captured by the defect potential.

[0006] In order to achieve the above technical objectives, the present invention provides a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization, which 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 substrate and the two-dimensional heterojunction material;

[0007] 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°.

[0008] In the technical solution provided by the present invention, a heterojunction structure is first constructed by "transition metal chalcogenide alloy-transition metal chalcogenide pure phase", and a controllable defect potential well is introduced by utilizing the chalcogenide vacancy defects rich in the chalcogenide alloy. Then, by controlling the torsion angle between the two, two torsion angles, small-angle quasi-coherent and large-angle quasi-incoherent, are set to achieve 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 constraints, and the defect IX is enhanced due to phonon coupling, the specific valley polarization state can be selectively enhanced by controlling the temperature of the heterojunction, thereby achieving tunable valley polarization.

[0009] As a preferred solution, the chemical formula of the two-dimensional transition metal chalcogenide is MX 2 , wherein M is a transition metal element and X is S or Se.

[0010] As a preferred solution, 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.

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

[0012] As a preferred solution, the fluorescence 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. The selection of the transition metal chalcogenide and the transition metal chalcogenide alloy must be strictly carried out in accordance with the above requirements. The non-overlapping fluorescence spectra of the two in this temperature range can effectively avoid the spectral interference between the intralayer excitons and the interlayer excitons, such as WS 2 The A exciton (~2.0 eV) and WS 0.9 Se 1.1 The defect state (~1.6 eV) forms a clear band offset, which is beneficial to the selective excitation and detection of interlayer excitons.

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

[0014] As a preferred solution, 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 a moiré potential trap.

[0015] As a preferred solution, 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.

[0016] The heterojunction material provided by the present invention adopts a double potential well competition regulation mechanism for the first time to achieve tunable localization of interlayer excitons. When the twist angle is at a small angle, the periodic moiré potential forms a deep potential well, and the excitons are localized as moiré IX. When the twist angle is at a large angle, the moiré potential period increases, causing the potential well to become shallow, the defect potential becomes dominant, and the excitons are localized as defect IX. Therefore, by adjusting the twist angle, reversible switching of the exciton capture mode can be achieved.

[0017] The present invention also discloses a method for preparing a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization. A single-layer two-dimensional transition metal chalcogenide compound and a single-layer two-dimensional transition metal chalcogenide compound alloy are obtained by mechanical stripping, and the two are stacked in sequence on a hard substrate according to a torsion angle, and annealed layer by layer to obtain the obtained material.

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

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

[0020] 1) The heterojunction material provided by the present invention is based on the synergistic effect of a large number of chalcogenide atom vacancy defects in a single-layer two-dimensional transition metal chalcogenide alloy and the torsion angle between the single-layer two-dimensional transition metal chalcogenide and the heterojunction material. The optical emission behavior of the interlayer excitons is jointly regulated by the chalcogenide atom vacancy defects and the moiré periodic potential of the heterojunction layer, thereby achieving tunable control of the localization of the interlayer excitons of the two layers. The dual-mode excitons of defect IX and moiré IX provided in the present invention can be parallel controlled by wavelength selection, which can significantly improve the integration of optoelectronic devices.

[0021] 2) In the preparation method provided by the present invention, a double potential well competition regulation mechanism is generated by stacking according to a specific twist angle, thereby achieving tunable interlayer exciton localization. Specifically, through analysis of the temperature-dependent PL spectrum and lifetime, the activation energies of 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 captured by the moiré potential, while at an interlayer twist angle of 3~22°, the interlayer excitons are captured by the defect potential.

[0022] 3) In the technical solution provided by the present invention, based on the dynamic characteristics of the phonon-assisted valley depolarization of the two interlayer excitons, the difference in valley dynamics between the defect interlayer excitons of the co-circularly polarized PL emission behavior and the moiré interlayer excitons of the cross-circularly polarized PL emission is analyzed through circularly polarized time-resolved PL spectroscopy, and specific valley polarization states can be selectively enhanced through temperature regulation. In addition, since the valley polarization rate of defect IX reaches 24.5% at 14 K under non-resonant excitation, it is greater than the "pure phase-pure phase TMD heterojunction" system in the prior art (the polarization rate is about 10% at 10K). BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the process for preparing single-layer materials and heterogeneous double-layer materials in the embodiments and comparative examples; the samples are annealed layer by layer.

[0024] Figure 2 Schematic diagram and morphology characterization diagram of the atomic structure in the heterogeneous double layer obtained in Example 1;

[0025] in, Figure 2 (a) Heterogeneous double-layer WS 2 / WS 0.9 Se 1.1 Schematic diagram of atomic structure and spatial distribution of defect-X, defect-IX and moiré-IX excitons, alloy monolayer WS 0.9 Se 1.1 The chalcogenide atomic vacancy defects in are marked by arrows; Figure 2 (b) WS 0.9 Se 1.1 Alloy and WS 2 / WS 0.9 Se 1.1 The simplified band structure of the heterolayer shows the defect-X, defect-IX and moiré-IX transitions. The electron in the defect level forms defect-X with the hole in the valence band. The defect-IX is formed by the transition of the electron in the defect level with the hole at the strong orbital hybrid Γ. WS 2 The electrons at the K point in the layer combine with the holes at the strong orbital hybridization Γ to form moiré-IX; Figure 2 (c) SiO 2 WS on Si substrate 2 / WS 0.9 Se 1.1 Bright-field optical image of the heterogeneous bilayer, with the inset showing the WS on the top of the sample 0.9 Se 1.1 and bottom WS 2 The polarization-resolved SHG signal of the monolayer region determined that the twist angle between the heterobilayer regions (HB) was about 6.4°; Figure 2(d) Atomic-resolution HAADF-STEM image of HB. The basic repeating unit of the moiré superlattice is about 3 nm, marked by a parallelogram. The inset is a single-layer WS. 0.9 Se 1.1 A magnified view of the alloy lattice, with the dashed arrow lines indicating the detected chalcogen atom vacancy defects;

[0026] Figure 3 A more detailed atomic structure of the heterogeneous double layer obtained in Example 1 and a morphology characterization diagram of the single-layer two-dimensional TMDs alloy;

[0027] in, Figure 3 (a) and Figure 3 (b) WS 2 / WS 0.9 Se 1.1 Heterogeneous bilayer and monolayer WS 0.9 Se 1.1 Atomic-resolution HAADF-STEM images of Figure 3 In (a), the moiré superlattice is marked by parallelograms, and the single-layer WS 0.9 Se 1.1 The interface between the alloy and the heterogeneous bilayer HB is marked by a line, and the inset is the WS 2 / WS 0.9 Se 1.1 FFT patterns of heterogeneous bilayers; Figure 3 (b), single-layer WS 0.9 Se 1.1 Typical chalcogen vacancy defects in alloys are marked with arrows; the inset is a single-layer WS 0.9 Se 1.1 FFT pattern of the alloy, d is the lattice constant; Figure 3 (c) is a single-layer WS 0.9 Se 1.1 EDS spectrum of the alloy;

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

[0029] in, Figure 4 (a) is a single-layer WS 0.9 Se 1.1 SEM surface scanning image of the alloy; Figure 4 (b) ~ Figure 4 (d) is a single-layer WS 0.9 Se 1.1 The element mapping of the alloy shows that the quantitative ratio of W, Se and S is 2724:2998:2209.

[0030] Figure 5WS of different interlayer twist angles provided in Example 1 and Example 2 2 / WS 0.9 Se 1.1 Heterogeneous bilayer and corresponding single-layer WS 2 and WS 0.9 Se 1.1 Room temperature Raman spectrum of

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

[0032] in, Figure 6 (a) HB (interlayer twist angle: 1.4° and 6.4°) and monolayer WS at 10 K (red curve) and 295 K (black dashed line) under 488 nm continuous wavelength (CW) laser excitation, respectively. 0.9 Se 1.1 and WS 2 Comparison of normalized PL spectra of regions, X 0 : neutral exciton; Figure 6 (b) Corresponding moiré-IX, defect-IX, defect-X, and WS under 640 nm pulse laser excitation 0.9 Se 1.1 -X and WS 2 -XNormalized time-resolved PL spectrum (TRPL) at 14 K;

[0033] Figure 7 For the single-layer WS in Comparative Example 3 0.9 Se 1.1 PL spectra of defect-X excitons in alloys depending on excitation power;

[0034] in, Figure 7 (a) is a single-layer WS 0.9 Se 1.1 Excitation power-dependent PL spectra of the alloys; Figure 7 (b) and Figure 7 (c) From Figure 7 (a) The extracted X 0 The relationship between the exciton peak energy and PL intensity of defect-X and the excitation power.

[0035] Figure 8 WS provided in Example 1 2 / WS 0.9 Se 1.1 Power-dependent PL spectra of defect-IX excitons in heterobilayers;

[0036] in, Figure 8 (a) is a comparison of PL spectra of HB regional defect-IX exciton emission in Example 1 at excitation powers of 0.1 μW and 40 μW; Figure 8 (b) PL spectrum of the defect-IX exciton emission dependent on the excitation power; Figure 8 (c) From Figure 8 (b) The excitation power dependence of the extracted and fitted defect-IX excitonic PL intensity and peak position.

[0037] Fig. 9 The power-dependent PL spectrum of the moiré-IX exciton in the heterogeneous bilayer provided in Example 2;

[0038] in, Fig. 9 (a) is a single-layer WS in the comparative example 0.9 Se 1.1 Alloy and WS 2 Bright field optical image of Fig. 9 (b) is the top single layer WS in the comparative example 0.9 Se 1.1 Alloy and bottom single layer WS 2 The polarization-dependent SHG signal of the region determines that the twist angle between the HB regions is about 1.4°; Fig. 9 (c) WS 2 / WS 0.9 Se 1.1 Power-dependent PL spectra of heterobilayers; Fig. 9 (d) Fig. 9 (c) The relationship between the extracted PL intensity and the excitation power.

[0039] Fig.10 The steady-state PL spectra of different exciton types in Example 1 and Example 2 vary with temperature;

[0040] in, Fig.10 (a) ~ Fig.10 (c) Temperature-dependent PL spectra of defect-X, defect-IX and moiré-IX exciton emissions at 3 μW excitation power; Fig.10 (d) ~ Fig.10 (f) Fig.10 (a) ~ Fig.10 (c) Temperature-dependent normalized PL spectrum. Fig.10 (g) ~ Fig.10 (i) Fig.10 (a) ~ Fig.10 (c) The extracted defects -X, X 0 , defect-IX and moiré-IX exciton PL intensities as a function of temperature.

[0041] Fig.11 The temperature-dependent circularly polarized PL spectra and the corresponding circular polarization degrees (ρ) of different interlayer excitons in Example 1 and Example 2;

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

[0043] Fig.12 For the single-layer WS in Comparative Example 3 0.9 Se 1.1 Temperature-dependent circularly polarization-resolved PL spectra of defect-X excitons in alloys;

[0044] in, Fig.12 (a) is a single-layer WS 0.9 Se 1.1 Evolution of circularly polarized PL spectra of defect-X in alloy; Fig.12 (b) Fig.12 (a) The corresponding relationship between valley polarization and energy.

[0045] Fig.13 WS of different interlayer twist angles provided in Example 1 and Example 2 2 / WS 0.9 Se 1.1 Temperature-dependent circularly polarization-resolved TRPL spectra of interlayer excitons in heterobilayers;

[0046] in, Fig.13 (a) and Fig.13 (b) Normalized circularly polarized time-resolved PL spectra of the excitons of defect IX and moiré-IX, respectively. All spectra were excited by a 640 nm laser with a power of 15 μW. Fig.13 (c) and Fig.13 (d) The dynamic evolution of valley polarization of defect-IX and moiré-IX at different temperatures, corresponding to Fig.13 (a) and Fig.13 (b) They increase the intervalley depolarization time with decreasing temperature. The curves are double exponential fit lines. The lifetime values ​​obtained from all fits are the averages of the two recombination lifetime components. Fig.13 (e) and Fig.13 (f) The mechanisms of the defect-IX and moiré-IX depolarization processes, respectively. The double-arrowed straight lines represent the depolarization with σ － (σ ＋ ) The width of the curve represents the number of radiated photons, and the solid arrows of the curve represent the carrier energy valley relaxation process, and the thickness of the arrows represents the amount of carrier relaxation.

[0047] Fig.14 Circularly polarized resolved TRPL spectrum of defect-X excitons in the single-layer alloy in comparative example 3 depending on temperature;

[0048] in, Fig.14 (a) ~ Fig.14 (c) is a single-layer WS 0.9 Se 1.1 Normalized circularly polarized time-resolved PL spectra of defect-X excitons in the alloy at 14 K, 30 K, and 50 K;

[0049] Fig.15 The power-dependent PL spectrum of defect-IX excitons in the heterogeneous bilayer provided in Comparative Example 1;

[0050] in, Fig.15 (a) is a single-layer WS in the comparative example 0.9 Se 1.1 Alloy and WS 2 Bright field optical image of Fig.15 (b) is the top single layer WS in the comparative example 0.9 Se 1.1 Alloy and bottom single layer WS 2 The polarization-dependent SHG signals of the regions determined that the twist angle between the HB regions was approximately 22.3°; Fig.15 (c) WS 2 / WS 0.9 Se 1.1 Power-dependent PL spectra of heterobilayers; Fig.15 (d) Fig.15 (c) The relationship between the extracted PL intensity and the excitation power. DETAILED DESCRIPTION

[0051] For ease of understanding of the present invention, the present invention will be described more fully 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, the purpose of providing these embodiments is 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 those generally understood by those skilled in the art of the present invention. The terms used in the specification 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 embodiment provides a two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization, and the preparation process is as follows:

[0054] 1) Mechanical stripping from bulk WS using tape 2 , WS 0.9 Se 1.1 A thin layer of material is peeled off from the source, and the tape is folded and torn three times to dissociate a thinner material. A PDMS film of about 1×1 cm is attached to the surface of one side of the glass slide, and the tape with the material is attached to the PDMS film; the tape is gently pressed to ensure that the material on the surface and the PDMS are in even and close contact, and after waiting for 3-5 minutes, the tape is peeled off from one side of the PDMS, and a part of the dissociated sample remains on the PDMS surface; the PDMS surrounding part containing the target sample in the single-layer area is cut off with a knife;

[0055] 2) Transfer the target sample on the PDMS surface to the 300 nm SiO 2 / Si substrate, and then stack another single layer sample up and down to form WS 2 / WS 0.9 Se 1.1 In the heterogeneous double-layer region, in order to ensure that the interlayer twist angle is 6-7°, the angle between the long sides of the upper and lower single-layer samples is used as a reference, and the angles of the microscope and the rotating transfer platform are combined to accurately control the parallelism of the long sides of the upper and lower single-layer samples. Finally, the prepared target heterogeneous double-layer sample is placed in a vacuum annealing furnace for annealing to reduce interlayer bubbles and impurities to enhance interlayer coupling. The vacuum degree in the furnace is maintained at 3.0×10 -5 mbar and the temperature was maintained at 130 °C for 6 hours to obtain a 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 double-layer material is not annealed.

[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 a single layer of WS precursor for preparing the heterogeneous double layer. 0.9 Se 1.1 alloy.

[0064] In order to better illustrate the characteristics and advantages of different excitons in the samples prepared by the present invention, the present invention also conducted a series of characterizations and tests on the materials obtained in the above embodiments and different comparative examples:

[0065] WS obtained in Example 1 and Example 2 2 / WS 0.9 Se 1.1 Heterojunction bilayer Figure 2 (a) shows a typical atomic structure diagram, in which a single layer of WS contains a large number of chalcogenide atomic vacancy defects. 0.9 Se 1.1 Alloy stacked on a single layer WS 2 Due to the existence of vacancy defects, the monolayer WS 0.9 Se 1.1 Electrons and holes in the alloy form defect-bound intralayer excitons (defect-X) under photoexcitation. 0.9 Se 1.1 Electrons in alloys and WS 2 When the holes in the heterogeneous bilayer are combined, defect interlayer excitons (defect-IX) can be formed. Such interlayer excitons can also be captured by the moiré potential to form moiré interlayer excitons (moiré-IX). In order to illustrate the distribution of defect-X, defect-IX and moiré-IX in the heterogeneous bilayer band arrangement, we refer to the research results of predecessors and draw a simplified schematic diagram of the band arrangement ( Figure 2 (b)). The valence band hole on the defect level in the single-layer alloy combines with the electron to form defect-X. WS 0.9 Se 1.1 The defect level electrons in the alloy layer and the holes at the Γ point of the heterojunction double layer hybridization form defect-IX with K-Γ transition, while WS 2The K-valley electrons of the layer and the holes on the Γ-point of the heterojunction double layer form moiré-IX. We used the PDMS-assisted site-directed transfer technique to deposit the moiré-IX on SiO 2 WS was prepared on a Si substrate. 2 / WS 0.9 Se 1.1 The heterogeneous bilayer was vacuum annealed to ensure clean contact between the interfaces ( Figure 2 Energy dispersive X-ray spectroscopy and scanning electron microscopy elemental mapping confirmed that the monolayer WS 0.9 Se 1.1 The composition of the alloy, such as Figure 3 and Figure 4 As shown. The Raman vibration modes in the heterogeneous bilayers of Example 1 and Example 2 are both red-shifted compared to the Raman vibration modes in the monolayer, which indirectly indicates that the interlayer coupling is strong, as shown in Figure 5 The stacking angle of the heterobilayer is determined by polarization-resolved SHG, as Figure 2 As shown in the inset of (c). In order to characterize the twisted WS of Example 1 2 / WS 0.9 Se 1.1 Atomic structures of heterobilayers and monolayer WS 0.9 Se 1.1 To investigate the defects in the alloy, we transferred the samples onto copper grids for high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements. Figure 2 (d) shows a WS with an interlayer twist angle of about 6.4° 2 / WS 0.9 Se 1.1 The moiré superlattice of the heterogeneous bilayer has a superlattice constant of about 3.0 nm ( Figure 3 The more detailed atomic structure features, such as lattice constants and interlayer torsion angles, are shown. The uncertainty between the theoretical values ​​and experimental data is about 2.5%. 0.9 Se 1.1 A more detailed STEM image of the chalcogenide atoms shows a clearly discernible hexagonal lattice of metal atoms, which contains many chalcogenide atomic vacancy defects, such as Figure 3 (b) and Figure 2 As shown in the inset in (d).

[0066] The present invention mainly studies the effect of chalcogen atom vacancies in the composite materials of Example 1 and Example 2 on the exciton state in the twisted TMDs heterobilayer, and performs steady-state PL spectrum analysis under continuous light excitation at a wavelength of 488 nm. Figure 6 By measuring the PL spectrum at 295 K, we observed a low-energy interlayer exciton emission peak in the heterojunction bilayer with twist angles of 1.4° and 6.4°, and a low-energy interlayer exciton emission peak in the monolayer WS0.9 S e1.1 Alloy and WS 2 The intralayer exciton X peak does not exist in the heterogeneous bilayer. At 10 K, the complex of intralayer exciton X in the single layer of the heterogeneous bilayer is almost quenched, and the intensity of IX is dominant. Three different IXs appear in the heterogeneous bilayer obtained in Example 2, with an energy spacing of about 60 meV, which is consistent with the moiré interlayer exciton feature. The IX in the heterogeneous bilayer in Example 1 is different from the single layer WS in Comparative Example 3. 0.9 Se 1.1 The energy of the low-energy peak observed in the alloy is consistent, and we infer that there is a close correlation between the two. The linear fitting coefficient of power-PL intensity (α = 0.24, ),like Figure 7 As shown, it is proved that the single-layer WS 0.9 Se 1.1 The strong low-energy broad peak up to 150 meV in the half-wavelength width (FWHM) in the alloy originates from the chalcogen atom vacancy defects, which is consistent with the existing literature. Due to the mutual repulsion of dipoles between excitons, the PL spectrum of interlayer excitons shows a significant blue shift with increasing power. The power-dependent PL intensity fitting in Example 1 shows that the intensity increases sublinearly with power (α=0.85), further proving that it may be captured by the defect potential ( Figure 8 As shown in Figure 2, by contrast, the interlayer excitons constrained by the moiré potential in Example 2 are only 0.78, indicating that the interlayer excitons are captured by a deeper moiré potential and are more difficult to delocalize from the potential ( Fig. 9 as shown).

[0067] The present invention then conducts temperature-dependent PL spectroscopy analysis on the embodiment and heterojunction example to further explore the possible relationship between defect-IX and the monolayer WS 0.9 Se 1.1 Possibility of defect-X-ray emission property correlation in alloys. Fig.10 (a) to (c) show the WS in the embodiment and the comparative example, respectively. 0.9 Se 1.1 Alloy single layer and WS 2 / WS 0.9 Se 1.1 Comparison of PL spectra of interlayer excitons in heterolayers. As the temperature increases, their emission quenching process and energy redshift trends are very similar, which is consistent with the temperature-dependent behavior of the semiconductor band gap. Due to the weakening of phonon interaction at low temperatures, the rate of interlayer charge transfer is increased, resulting in defect-IX ( Fig.10 (b)) and Mohr-IX ( Fig.10 (c)) is much stronger than the defect-X ( Fig.10 (a)). As shown in the intensity-normalized temperature-dependent PL spectrum ( Fig.10(d), 10 (e)), the PL intensity of defect-X and defect-IX is completely quenched at about 80 K, which may mean the existence of similar defects. The PL intensity of moiré-IX is quenched around 200 K and transforms from multiple peaks to a single peak as the temperature increases ( Fig.10 (c), 10(f)), which shows that the interlayer excitons are indeed trapped by the moiré potential.

[0068] Fig.10 (g) to (i) show the Fig.10 The PL intensity of different excitons extracted from (a) to (c) varies with temperature. We use the Arrhenius equation To analyze their radiative and non-radiative processes with temperature changes, is the Boltzmann constant, is the activation energy, and A is proportional to the thermal dissociation rate. After multiple fitting calculations and averaging, the activation energy of defect-X and defect-IX is almost the same, about 20 meV ( Fig.10 (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 the chalcogen atom vacancies. Alloy monolayer WS 0.9 Se 1.1 The activation energy of the neutral exciton (103 meV) is much greater than the activation energy of the defect-X, indicating that the defect-X cannot appear at room temperature ( Fig.10 (g)). Fig.10 (i) shows that the activation energy of mole-IX is 22.3 meV, which is larger than that of defect-IX.

[0069] The present invention further studies the valley polarization dynamics of interlayer excitons captured by defects and moiré potential at different temperatures in Example 1 and Example 2. Valley polarization and exciton lifetime ( ) and valley depolarization time ( ), we can use the formula To understand, is the polarization degree without valley relaxation. Fig.13 (a) and (b) show the circularly polarized-resolved PL decays of moiré-IX and defect-IX under 640 nm pulsed laser excitation, respectively. As the temperature increases from 14 K to 50 K, the moiré-IX and defect-IX and The difference between the components decreases, which is consistent with Fig.11 (a) and 11 (c) are consistent. When the temperature increases from 14 K to 50 K, the defect-IX and The component lifetimes 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, respectively. Similarly, the monolayer WS 0.9 Se 1.1 The alloy defect-X circular polarization degree is close to 0 ( Fig.12 ),That and The component lifetimes all decrease from about 56 ns at 14 K to about 22 ns at 50 K, e.g. Fig.14 As shown in Figure 2, 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 larger momentum space mismatch of the defect exciton, which leads to the longer exciton lifetime.

[0070] In order to better understand the generation mechanism of valley polarization in the present invention, we and The lifetime of the component calculates the change in valley polarizability over time and is defined as ,in and They are and PL intensity of a component at time t. Fig.13 (c) and (d) summarize the changes of valley polarization with time at different temperatures. When the temperature rises from 14 K to 50 K, the initial valley polarization of defect-IX and moiré-IX at time 0 When the temperature increases from 14 K to 40 K, the valley depolarization lifetime of defect-IX decreases from 0.24 to 0 and from 0.35 to 0.15, respectively. From 10.5 ns to 2.2 ns. Compared with defect-IX, moiré-IX has a shorter exciton lifetime. and longer valley depolarization lifetime ( from 52 ns at 14 K to about 2.3 ns at 50 K), leading to a larger valley polarization ρ. 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 ( Fig.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 ( Fig.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. Fig.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 are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation modes, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are protected by 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 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°.

2. The two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization according to claim 1, characterized in that: The general chemical formula of the two-dimensional transition metal chalcogenide is MX2, wherein M is a transition metal element and X is S or Se.

3. The two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization according to claim 1, characterized in that: 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.

4. The two-dimensional heterojunction material with tunable interlayer exciton localization and valley polarization according to claim 3, characterized in that: 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.

5. 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.

6. 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.

7. 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.

8. 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 7, 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.

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

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