Method for enhancing TMDs fluorescence based on dark state energy transfer mechanism and application thereof

Abstract

The invention belongs to the field of low-dimensional semiconductor material micro-nano photoelectric integration, and particularly relates to a method for enhancing TMDs fluorescence based on a dark state energy transfer mechanism, a perovskite-TMDs heterojunction is obtained, then excitation is performed through a two-photon pump light source, dark state energy is accumulated, and space-prohibited dark state energy transfer is induced, so that the TMDs fluorescence is enhanced; the perovskite-TMDs heterojunction comprises a perovskite nanosheet and TMDs compounded on the surface of the perovskite nanosheet, and dark exciton state energy bands of perovskite and TMDs are matched. The invention also provides application of the method to improvement of fluorescence intensity in a device. The invention provides a brand new TMDs fluorescence enhancement thought, the TMDs and the perovskite are innovatively used for constructing a heterojunction system, and through combined control of heterojunction energy band matching components constructed by the TMDs and the perovskite and excitation of the two-photon pump light source, dark-state energy transfer can be induced, and the fluorescence enhancement effect of the two-photon pump light source is improved. Therefore, fluorescence enhancement of TMDs is realized based on a dark state energy transfer mechanism.

Description

Technical Field

[0001] The present invention relates to a method for enhancing WS2 fluorescence based on a dark-state energy transfer mechanism and application thereof, and belongs to the field of micro-nano optoelectronic integration of low-dimensional semiconductor materials. Background Art

[0002] Excellent semiconductor materials are the key foundation for building multifunctional and efficient new optoelectronic devices. TMDs have great research potential in exploring new nano-optoelectronic devices due to their excellent optoelectronic properties. However, due to the ultra-thin atomic layer thickness, it cannot absorb enough light to bring about effective light-matter interaction, and the single-layer fluorescence quantum yield is very low. The optoelectronic performance based on TMDs still has a lot of room for improvement. Therefore, the rational selection of another semiconductor material to couple with it to construct a van der Waals heterojunction is a very feasible and important scientific means to achieve fluorescence enhancement of TMDs and design high-performance optoelectronic devices.

[0003] The optical properties of TMDs are determined by excitons composed of electron-hole pairs bound by Coulomb interactions. The fine structure of excitons gives rise to a variety of exciton emission characteristics in semiconductor materials. This complex exciton emission causes TMDs to exhibit non-uniform optical properties with low fluorescence quantum yield. In response to these shortcomings, researchers currently mainly enhance the fluorescence emission of TMDs through physical adsorption, chemical adsorption, point doping, stress tuning and other means. It is worth noting that the dipole-forbidden dark exciton state occupies the main group distribution in the exciton distribution, and most excitons relax to the dark state, thereby limiting the photoluminescence (PL) efficiency of organic or inorganic semiconductors. This also means that dark exciton states have the potential to store energy because they have a very long lifetime and a large number, which in principle can provide an excellent platform for energy harvesting for solar cells and light-emitting devices. Using energy resonance to transfer and recombine the dark state exciton pool into bright exciton emission can significantly improve the photoelectric conversion efficiency of optoelectronic devices.

[0004] However, the optical inaccessibility of dark excitons poses a huge challenge to their detection and application in optoelectronic devices. Although their detection can be enhanced using various rigorous techniques, such as applying ultrastrong magnetic fields, coupling with surface plasmon excitons, or building complex optical cavities, there is a large research gap in the use of dark excitons as energy storage to improve the optoelectronic properties of semiconductor materials. Therefore, finding a simple and universal strategy to effectively utilize the energy stored in dark exciton states remains an unresolved scientific problem.

[0005] In summary, dark exciton states show great potential in condensed matter physics and optoelectronics due to their long lifetime and rich band structure distribution. However, their optical transition forbidden characteristics severely limit their experimental exploration and hinder their application. Summary of the invention

[0006] The first purpose of the present invention is to provide a method for enhancing TMDs fluorescence based on a dark state energy transfer mechanism, aiming to reveal the physical mechanism of dark state energy transfer enhancing TMDs fluorescence emission under two-photon excitation.

[0007] The second purpose of the present invention is to provide the optoelectronic application in the near-infrared field of enhancing TMDs fluorescence based on the dark state energy transfer mechanism.

[0008] Due to the optical transition prohibition problem of dark exciton states, the current research on the brightening of dark exciton states mainly relies on very strict technology, and there is a gap in the industry in exploring the energy application of dark exciton states. In addition, the existing excitation methods of TMDs will reduce the fluorescence intensity of TMDs to a certain extent. In response to this technical problem, the present invention provides the following solutions:

[0009] A method for enhancing TMDs fluorescence based on a dark state energy transfer mechanism, obtaining a perovskite-TMDs heterojunction (also referred to as a perovskite-TMDs composite material in the present invention), followed by excitation by a two-photon pumping light source, accumulating dark state energy and inducing parity-forbidden dark state energy transfer, thereby enhancing TMDs fluorescence;

[0010] The perovskite-TMDs heterojunction comprises a perovskite nanosheet and TMDs compounded on the surface thereof, wherein the dark exciton state energy bands of the perovskite and TMDs match.

[0011] The present invention provides a new idea for TMDs fluorescence enhancement, which innovatively constructs a heterojunction system with TMDs and perovskites, and jointly controls the dark exciton state energy band matching components of the heterojunction constructed by TMDs and perovskites and the excitation of the two-photon pump light source, thereby inducing dark state energy transfer, thereby achieving fluorescence enhancement of TMDs based on the dark state energy transfer mechanism.

[0012] In the present invention, under the pumping of two-photon excitation light, the deep dark exciton state energy in the donor perovskite is activated and accumulated in large quantities, and the donor dark state energy is further transferred to the matching dark state energy level in the acceptor TMDs by means of resonant energy transfer, and finally the fluorescence emission of TMDs is enhanced. In the present invention, the combination of the perovskite-TMDs composite material with matching dark exciton state energy band and the two-photon pumping light source means is the key to enhancing the fluorescence of TMDs based on the new dark state energy transfer mechanism. On this basis, the research of the present invention also shows that the joint control of the thickness of the perovskite nanosheet is helpful to further regulate the effect of enhancing the fluorescence of TMDs based on the dark state energy transfer mechanism.

[0013] Preferably, the thickness of the perovskite nanosheet is below 250nm, preferably 35 to 220nm, and more preferably 130 to 180nm. The present invention shows that the control of the perovskite nanosheet within the preferred thickness range can further synergistically improve the performance of the dark state energy transfer mechanism to enhance the fluorescence of TMDs. For example, within the preferred range, the fluorescence enhancement of the monolayer WS2 can exceed one order of magnitude.

[0014] The technical solution of the present invention has excellent universality, and the composition of the perovskite-TMDs heterojunction only needs to satisfy the dark exciton state energy band matching.

[0015] For example, in the embodiments of the present invention, the perovskite nanosheet is at least one of a CsPbX3 (X=Cl, Br, I) nanosheet or a two-dimensional RP phase perovskite; further, it can be CsPbBr3.

[0016] In the present invention, the perovskite nanosheets can be synthesized based on conventional means, for example, the perovskite nanosheets can be prepared by CVD.

[0017] An optional solution of the present invention is to prepare the CsPbBr3 nanosheets by a CVD method, the steps of which are: heating and volatilizing a mixed powder of CsBr and PbBr2, and chemically depositing the CsPbBr3 nanosheets on the surface of a mica substrate with the assistance of a carrier gas.

[0018] The molar ratio of CsBr to PbBr2 is 1:1 to 1.1.

[0019] The heating temperature of the mixed powder is 550-580°C.

[0020] The mica substrate is located downstream of the mixed powder at a distance of 20 to 25 cm.

[0021] The carrier gas is at least one of inert gases.

[0022] During the chemical deposition stage, the flow rate of the carrier gas is 10 to 100 sccm.

[0023] The chemical deposition time is 10 to 100 minutes.

[0024] In the present invention, the van der Waals heterojunction constructed by perovskite and TMDs is a type II energy band arrangement.

[0025] In the present invention, the TMDs are two-dimensional materials of at least one of sulfides and selenides of at least one element of W and Mo.

[0026] Furthermore, the TMDs is WS2.

[0027] In the present invention, the TMDs are compounded on the surface of the perovskite nanosheets through a mechanical exfoliation method and a dry transfer technology.

[0028] For example, TMDs can be peeled off on PMMA and then dry-transferred to perovskite nanosheets to obtain the perovskite-TMDs heterojunction.

[0029] In another embodiment of the present invention, in the perovskite-TMDs heterojunction, an insulating spacer layer is further compounded between the perovskite and the TMDs (constituting a perovskite-insulating barrier layer-TMDs composite system).

[0030] In the present invention, the insulating spacer layer is a transparent material; further, hexagonal boron nitride (h-BN).

[0031] In the present invention, a mechanical exfoliation method and a dry transfer technique are used to stack and insulate spacer layers on the surface of perovskite nanosheets.

[0032] In the present invention, the thickness of the insulating spacer layer is below 27 nm.

[0033] In the present invention, the TMDs and the optional insulating isolation layer in the perovskite-TMDs composite material and the perovskite-insulating barrier layer-TMDs composite material can be transferred by dry method to construct a vertically stacked van der Waals heterojunction.

[0034] In the present invention, the carrier population inside WS2 can be enhanced through dark state energy transfer under two-photon excitation, thereby enhancing the near-infrared photoelectric response.

[0035] In the present invention, the two-photon pumping light source is a femtosecond pulse laser with a wavelength of 800 to 1080 nm.

[0036] Preferably, the two-photon pump light source has a repetition frequency of 1 MHZ and a pulse width of 80 to 120 fs.

[0037] The present invention also provides a device, which comprises the perovskite-TMDs heterojunction described in the method described in the present invention and utilizes the method to enhance TMDs fluorescence.

[0038] Beneficial Effects

[0039] 1. The present invention has successfully developed a method for enhancing TMDs fluorescence based on the dark-state energy transfer mechanism and its application, filling a gap in the industry.

[0040] 2. The present invention has found that based on the pumping of two-photon excitation light, a large amount of dark exciton state energy in the band-matched perovskite nanosheets is fully activated and transferred in the 3p dark exciton state of the monolayer TMDs that matches its energy band through the resonance energy transfer mechanism, thereby enhancing the fluorescence of the monolayer TMDs and improving the photoelectric response of the monolayer TMDs near-infrared detector.

[0041] 3. The present study also found that by controlling the thickness of perovskite nanosheets, the dark state energy transfer efficiency can be artificially controlled, and the fluorescence enhancement effect of single-layer TMDs can be successfully controlled, providing important guidance for the subsequent design of more flexible and widely applicable near-infrared optoelectronic devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Attached Figure 1 Depend on Figure 1 a- Figure 1 c composition;

[0043] Attached Figure 1 a is a schematic diagram of the CsPbBr3 nanosheets prepared in Example 1;

[0044] Attached Figure 1 b and 1c are CsPbBr3 nanosheets with controllable thickness prepared in Example 1;

[0045] Attached Figure 2 Depend on Figure 2 a- Figure 2 e composition;

[0046] Attached Figure 2 a is an optical photograph of the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 2 and a PL intensity mapping of CsPbBr3 nanosheets and monolayer WS2 under 800nm ​​excitation light pumping, with a scale bar of 10μm;

[0047] Attached Figure 2 b is a schematic diagram of the dark state energy transfer mechanism of the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 2 in the type II energy band arrangement;

[0048] Attached Figure 2 c is the PL and absorption spectra of each component in the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 2;

[0049] Attached Figure 2 d is the PL spectrum comparison between the heterojunction region and the pure material in the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 2;

[0050] Attached Figure 2e is a comparison of the fluorescence lifetimes of the heterojunction region and the pure material in the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 2; the inset is a comparison of the rising edges of the lifetimes of CsPbBr3 and WS2 in the heterojunction region;

[0051] Attached Figure 3 Depend on Figure 3 a- Figure 3 e composition;

[0052] Attached Figure 3 a is the PL intensity mapping of the single-layer WS2 of the CsPbBr3 / h-BN / WS2 van der Waals heterojunction prepared in Example 3 under 800nm ​​excitation light pumping, the scale bar is 10μm;

[0053] Attached Figure 3 b is the PL spectrum comparison between the heterojunction region and the pure material in the CsPbBr3 / h-BN / WS2 van der Waals heterojunction prepared in Example 3;

[0054] Attached Figure 3 c is the comparison of the fluorescence lifetime of CsPbBr3 in the heterojunction region and pure CsPbBr3 in the CsPbBr3 / h-BN / WS2 van der Waals heterojunction prepared in Example 3;

[0055] Attached Figure 3 d is the comparison of the fluorescence lifetime of WS2 in the heterojunction region and pure WS2 in the CsPbBr3 / h-BN / WS2 van der Waals heterojunction prepared in Example 3;

[0056] Attached Figure 3 e The dependence of the energy transfer rate in the CsPbBr3 / h-BN / WS2 van der Waals heterojunction prepared in Example 3 on the thickness of h-BN, and the inset is a schematic structural diagram of the CsPbBr3 / h-BN / WS2 van der Waals heterojunction;

[0057] Attached Figure 4 Depend on Figure 4 a- Figure 4 f composition;

[0058] Attached Figure 4 a is an optical photograph of the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 4;

[0059] Attached Figure 4 b is the PL intensity mapping of the single-layer WS2 of the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 4 under 800nm ​​excitation light pumping, the scale bar is 10μm;

[0060] Attached Figure 4c is the comparison of PL spectra of single-layer WS2 at different positions of the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 4 under 800nm ​​excitation light pumping;

[0061] Attached Figure 4 d is the comparison of the lifetime of single-layer WS2 at different positions of the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 4 under 800nm ​​excitation light pumping;

[0062] Attached Figure 4 e is the dependence of the WS2 PL enhancement factor on the thickness of the CsPbBr3 nanosheets in the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 4;

[0063] Attached Figure 4 f is a schematic diagram of energy transfer of the CsPbBr3 / WS2 van der Waals heterojunction prepared in Example 4;

[0064] Attached Figure 5 Depend on Figure 5 a- Figure 5 c composition;

[0065] Attached Figure 5 a is an optical photograph of the BAI / WS2 van der Waals heterojunction prepared in Example 5;

[0066] Attached Figure 5 b is the PL intensity mapping of the single-layer WS2 of the BAI / WS2 van der Waals heterojunction prepared in Example 5 under 800nm ​​excitation light pumping;

[0067] Attached Figure 5 c is the comparison of the fluorescence lifetime between the heterojunction region and the pure material in the BAI / WS2 van der Waals heterojunction prepared in Example 5;

[0068] Attached Figure 6 Depend on Figure 6 a- Figure 6 b composition;

[0069] Attached Figure 6 a is an optical photograph of the CsPbBr3 / WS2 van der Waals heterojunction photodetector prepared in Example 6, and the inset is a schematic diagram of its structure;

[0070] Attached Figure 6 b is the comparison of the photocurrent data of WS2 in the heterojunction region and pure WS2 of the CsPbBr3 / WS2 van der Waals heterojunction photodetector prepared in Example 6 under 808nm two-photon excitation light pumping;

[0071] Attached Figure 7 Depend on Figure 7 a- Figure 7 c composition;

[0072] Attached Figure 7 a is an optical photograph of the CsPbBr3 / WS2 van der Waals heterojunction prepared in Comparative Example 1 and a PL intensity mapping of CsPbBr3 nanosheets and monolayer WS2 under 488nm excitation light pumping, with a scale bar of 10μm;

[0073] Attached Figure 7 b is the PL spectrum comparison between the heterojunction region and the pure material in the CsPbBr3 / WS2 van der Waals heterojunction prepared in Comparative Example 1;

[0074] Attached Figure 7 c is a comparison of the fluorescence lifetime of the heterojunction region and the pure material in the CsPbBr3 / WS2 van der Waals heterojunction prepared in Comparative Example 1; the inset is a schematic diagram of the carrier transfer mechanism in the type II band arrangement;

[0075] Attached Figure 8 Depend on Figure 8 a- Figure 8 h composition;

[0076] Attached Figure 8 a is the optical photograph of the CsPbBr3 / WSe2 van der Waals heterojunction prepared in Comparative Example 2 and the PL intensity mapping of the CsPbBr3 nanosheet and monolayer WSe2 under 488nm and 800nm ​​excitation light pumping respectively;

[0077] Attached Figure 8 b is the PL spectrum comparison between the CsPbBr3 / WSe2 van der Waals heterojunction prepared in Comparative Example 2 and the pure material under 488nm excitation;

[0078] Attached Figure 8 c is the PL spectrum comparison between the heterojunction region and the pure material of the CsPbBr3 / WSe2 van der Waals heterojunction prepared in Comparative Example 2 under 800nm ​​excitation;

[0079] Attached Figure 8 d is the absorption spectrum of the WSe component in the CsPbBr3 / WSe2 van der Waals heterojunction prepared in Comparative Example 2;

[0080] Attached Figure 8 e is the comparison of the fluorescence lifetime of CsPbBr3 in the heterojunction region and pure CsPbBr3 under 400nm excitation of the CsPbBr3 / WSe2 van der Waals heterojunction prepared in Comparative Example 2;

[0081] Attached Figure 8 f is the comparison of the fluorescence lifetime of WSe2 in the heterojunction region and pure WSe2 under 400nm excitation of the CsPbBr3 / WSe2 van der Waals heterojunction prepared in Comparative Example 2;

[0082] Attached Figure 8 g is the comparison of the fluorescence lifetime of CsPbBr3 in the heterojunction region and pure CsPbBr3 under 800nm ​​excitation of the CsPbBr3 / WSe2 van der Waals heterojunction prepared in Comparative Example 2;

[0083] Attached Figure 8 h is the comparison of the fluorescence lifetime of WSe2 in the heterojunction region and pure WSe2 under 800nm ​​excitation of the CsPbBr3 / WSe2 van der Waals heterojunction prepared in Comparative Example 2;

[0084] Attached Fig. 9 Depend on Fig. 9 a- Fig. 9 b composition;

[0085] Attached Fig. 9 a is an optical photograph of the CsPbBr3 / WS2 van der Waals heterojunction photodetector prepared in Comparative Example 3, and the inset is a schematic diagram of its structure;

[0086] Attached Fig. 9 b is the comparison of the photocurrent data of WS2 in the heterojunction region and pure WS2 of the CsPbBr3 / WS2 van der Waals heterojunction photodetector prepared in Comparative Example 3 under 450nm two-photon excitation light pumping;

[0087] from Figure 1 It can be seen that the CVD method for preparing CsPbBr3 nanosheets adopts a tube-in-tube one-step method, and the deposition is carried out under an argon environment;

[0088] from Figure 2 It can be seen that the fluorescence lifetime of CsPbBr3 in the CsPbBr3 / WS2 van der Waals heterojunction will be quenched under 800nm ​​two-photon excitation, while the fluorescence and lifetime of WS2 in the heterojunction region will be greatly improved; at the same time, there is an energy band overlap between the dark state of CsPbBr3 and the 3p dark state of WS2, and under two-photon excitation, parity prohibition will lead to the dominance of the energy transfer process;

[0089] from Figure 3 It can be seen that by statistically analyzing the energy transfer efficiency of a series of CsPbBr3 / h-BN / WS2 van der Waals heterojunctions based on the insulating spacer h-BN with different thicknesses, the present invention can determine the physical mechanism that promotes the fluorescence enhancement of WS2 as follows: Resonance energy transfer, and belongs to the 2D-2D category;

[0090] from Figure 4 It can be seen that by statistically analyzing the fluorescence enhancement factors of WS2 based on CsPbBr3 with different thicknesses in a series of CsPbBr3 / WS2 van der Waals heterojunctions, the present invention can regulate the upconversion fluorescence output of WS2 by controlling the thickness of CsPbBr3;

[0091] from Figure 5 It can be seen that by replacing BAI perovskite with a band structure similar to that of CsPbBr3 and WS2 to construct a van der Waals heterojunction, the fluorescence enhancement of WS2 can also be promoted by inducing dark exciton state energy transfer under two-photon excitation, indicating that this mechanism has a certain universality in van der Waals heterojunctions with matching dark exciton state energy bands;

[0092] from Figure 6 It can be seen that the photodetector constructed based on the dark state energy transfer mechanism in the CsPbBr3 / WS2 van der Waals heterojunction, under 808nm two-photon excitation light pumping, the near-infrared photocurrent of the single-layer WS2 in the heterojunction region will be enhanced by one order of magnitude compared with pure WS2;

[0093] from Figure 7 It can be seen that under 488nm single-photon excitation, the CsPbBr3 / WS2 van der Waals heterojunction will undergo carrier transfer caused by the II band arrangement, resulting in double quenching of CsPbBr3 and WS2 in the heterojunction region;

[0094] from Figure 8 It can be seen that in the CsPbBr3 / WSe2 van der Waals heterojunction with mismatched dark state energy bands, dark state energy transfer cannot occur and the fluorescence of WSe2 cannot be enhanced;

[0095] from Fig. 9 It can be seen that the photocurrent of the photodetector built by CsPbBr3 / WS2 van der Waals heterostructure under 450nm single-photon excitation is only 2 times higher than that of pure WS2. DETAILED DESCRIPTION

[0096] The two-photon steady-state test excitation light source and the transient test excitation light source are 800nm ​​femtosecond lasers with a repetition frequency of 1MHz and a pulse width of 80-120fs.

[0097] The single-photon steady-state test excitation light source and transient test excitation light source are 488nm, 450nm continuous laser and 400nm femtosecond laser respectively, with a repetition frequency of 1MHz and a pulse width of 80-120fs.

[0098] The fluorescence testing instrument is a confocal microscope system, model: WITec, alpha-300.

[0099] The transient test instrument is a streak camera, model: C10910, Hamamatsu.

[0100] The material thickness testing instrument is an atomic force microscope, model: Dimension Icon.

[0101] The present invention provides a new method for improving the fluorescence of TMDs based on the dark state energy transfer mechanism by irradiating a perovskite-TMDs heterojunction with a two-photon pumping light source. The technical solution has good universality for perovskite-TMDs. If the two can basically meet the dark exciton state energy band matching characteristics, the fluorescence enhancement of TMDs can be innovatively achieved based on the new mechanism of the present invention.

[0102] For example, as an optional embodiment, the perovskite-TMDs heterojunction takes CsPbBr3-WS2 as an example.

[0103] By further studying this van der Waals heterojunction system, the present invention successfully realized a method for improving the fluorescence emission of a single layer of WS2 based on a dark state energy transfer mechanism and its optoelectronic application, and achieved controllable energy transfer efficiency and WS2 fluorescence enhancement factor by adjusting the thickness of the spacer layer h-BN and the thickness of the CsPbBr3 nanosheets.

[0104] The technical solution of the present invention innovatively discovers that two-photon pumping can effectively accumulate dark exciton state energy, and under the synergistic effect of parity prohibition and resonance energy transfer mechanism, it promotes the transfer of rich dark state energy in CsPbBr3 to the 3p dark state of WS2, thereby greatly enhancing the upconversion fluorescence emission of monolayer WS2.

[0105] Preferably, the technical solution of the present invention, based on the efficient dark state energy transfer mechanism, can increase the upconversion fluorescence emission of the monolayer WS2 by an order of magnitude.

[0106] Preferably, the technical solution of the present invention, based on the efficient dark state energy transfer mechanism, can improve the near-infrared photoelectric response of the single-layer WS2 by an order of magnitude.

[0107] Preferably, the technical solution of the present invention, based on the efficient dark state energy transfer mechanism, can extend the energy transfer distance to more than 200 nm.

[0108] In the present invention, there are no special requirements for the CVD synthesis method and conditions of the CsPbBr3 nanosheets, which can be achieved based on existing equipment and theory to obtain nanosheets of the required thickness.

[0109] For example, CsBr and PbBr2 powder raw materials are chemically vapor deposited in a carrier gas.

[0110] Preferably, the molar ratio of CsBr and PbBr2 powders is 1:1.

[0111] Preferably, the heating temperature of the source powder (raw material powder) is, for example, 550-580° C. The distance between the substrate and the source powder is, for example, 20-25 cm.

[0112] Preferably, the carrier gas for chemical vapor deposition is an inert gas, such as argon.

[0113] In the present invention, the planar shape of the CrPbBr3 nanosheet may be, for example, a rectangle, and the planar size thereof may be, for example, several to several tens of micrometers.

[0114] In the present invention, the two-dimensional materials WS2 and h-BN are both prepared by mechanical exfoliation.

[0115] In the present invention, the van der Waals heterojunction is prepared by dry transfer technology.

[0116] In the present invention, the two-photon pumping light source is, for example, an 800nm ​​femtosecond pulse laser with a repetition frequency of 1MHZ and a pulse width of 80 to 120fs.

[0117] The present invention found that the controllable synthesis of thickness-dependent CsPbBr3 nanosheets can achieve artificial regulation of the dark state energy transfer effect, and further realize the controllable modulation of WS2 upconversion fluorescence enhancement.

[0118] In the present invention, the thickness of the controllable CsPbBr3 nanosheets ranges from 35 to 250 nm.

[0119] Preferably, when the thickness of the CrPbBr3 nanosheet is 157 nm, the upconversion fluorescence enhancement of the single-layer WS2 is the largest, and the maximum enhancement factor is 12.1.

[0120] In the following cases, unless otherwise stated, the steps for preparing CsPbBr3 nanosheets by CVD method are as follows:

[0121] A mixed powder of CsBr and PbBr2 with a molar ratio of 1:1 is placed in the center of a quartz tube, and a mica substrate is placed 20-25 cm downstream of the powder source. After the quartz tube is loaded into a tube furnace, it is purged with high-purity argon gas at a flow rate of 500 sccm for 5 minutes, and then the furnace is slowly heated to 550-580°C (source temperature) and kept at a constant temperature for 75 minutes. CsPbBr3 nanosheets of different thicknesses are cultivated under a gas flow of 30 sccm. During the growth process, the pressure in the tube is kept at atmospheric pressure. Finally, the tube furnace is naturally cooled to room temperature to obtain CsPbBr3 nanosheets with a rectangular plane and a thickness range of 35-250 nm.

[0122] Example 1

[0123] CsPbBr3 can be obtained by chemical vapor deposition based on conventional single-temperature zone deposition equipment. Figure 1As shown in a, a mixed powder of CsBr and PbBr2 with a molar ratio of 1:1 is placed in the center of the quartz tube, and a mica substrate is placed 20-25 cm downstream of the powder source. After the quartz tube is loaded into the tube furnace, it is purged with high-purity argon gas with a flow rate of 500 sccm for 5 minutes, and then the furnace is slowly heated to 550-580°C (source temperature) and kept at a constant temperature for 75 minutes. CsPbBr3 nanosheets of different thicknesses are cultivated under a 30 sccm Ar gas flow. During the growth process, the pressure in the tube is kept at atmospheric pressure. Finally, the tube furnace is naturally cooled to room temperature to obtain CsPbBr3 nanosheets with a rectangular plane and a thickness range of 35-250nm ( Figure 1 b and Figure 1 c). The morphology and size of the grown CsPbBr3 can be controlled by comprehensively adjusting the temperature of the central temperature zone of the tube furnace and the substrate placement position. The thickness of the deposition can be regulated by controlling the central temperature (source temperature) and the distance between the substrate and the source. For example, the higher the temperature and / or the closer the distance between the substrate and the source, the thicker the material. Lowering the temperature within the range and / or extending the distance between the source and the substrate can help reduce the thickness of the two-dimensional material. In the present invention, under the conditions described above, CsPbBr3 nanosheets with a thickness of 35 to 250 nm can be obtained, and since the quantum confinement effect that can change the band structure only acts on materials with a size of less than 10 nm, the band structure of the 35 to 250 nm CsPbBr3 nanosheets prepared by the present invention is basically the same.

[0124] Example 2

[0125] Study on dark state energy transfer of “CsPbBr3 / WS2” van der Waals heterojunction:

[0126] like Figure 2 As shown in a, a van der Waals heterojunction is prepared by vertically stacking WS2 using dry transfer technology on the basis of CsPbBr3 nanosheets deposited on mica sheets, with a scale of 10μm. The specific preparation method is as follows: first, the WS2 bulk material is adhered to the tape, and it is dispersed by pasting it with different tapes multiple times; then, a PDMS of appropriate size is selected and fixed on a glass slide, and the tape carrying the WS2 bulk source after dispersion is pasted on the PDMS, and the tape is slowly torn off after standing for about 10 minutes to obtain a single layer of WS2 prepared by mechanical stripping; finally, the glass slide is flipped and fixed on a three-dimensional transfer table, and after positioning under an optical microscope, it is pressed onto the target CsPbBr3 nanosheet to complete the preparation of the van der Waals heterojunction.

[0127] Then, the PL intensity mapping test was performed using 800nm ​​two-photon excitation light pumping. The heterojunction region showed CsPbBr3 fluorescence quenching and monolayer WS2 fluorescence enhancement ( Figure 2d), and the transient fluorescence test also shows that the lifetime of CsPbBr3 in the heterojunction region is shortened but the lifetime of the single-layer WS2 is prolonged, and the rise of WS2 is slowed down ( Figure 2 e). In summary, the absorption spectrum comparison ( Figure 2 c) and schematic diagram of energy band arrangement ( Figure 2 b), it can be determined that in the van der Waals heterojunction system, the dominant dynamic mechanism is the resonant energy transfer between the dark state of CsPbBr3 and the 3p dark state of WS2.

[0128] Example 3

[0129] Compared with Example 2, the only difference is that an insulating spacer layer h-BN is inserted into the "CsPbBr3 / WS2" van der Waals heterojunction, and the thickness of h-BN is changed, specifically: 0, 5.5, 8.5, 9.2, 11.5, 17.5, 26.9 nm.

[0130] like Figure 3 As shown in a, h-BN of different thicknesses are vertically stacked onto CsPbBr3 by dry transfer technology to form CsPbBr3 / h-BN / WS2 van der Waals heterojunctions, scale bar 10μm. Figure 3 b- Figure 3 d shows the upconversion fluorescence spectrum and lifetime changes of CsPbBr3 and WS2 in the heterojunction region of h-BN with different thicknesses. The thinner the insulating spacer layer h-BN, the more severe the quenching of CsPbBr3, while the fluorescence enhancement of WS2 is greater. Figure 3 e summarizes the dependence of energy transfer efficiency on the donor-acceptor distance in multiple van der Waals heterojunctions with different h-BN thicknesses, where the donor-acceptor distance = the thickness of the donor CsPbBr3 unit / 2 + the thickness of the h-BN + the thickness of the acceptor monolayer WS2 / 2. The blue fitting curve reveals that this energy transfer mechanism conforms to the 2D-2D category. Resonance Energy Transfer.

[0131] Example 4

[0132] Compared with Example 2, the only difference is that the thickness of the CsPbBr3 nanosheets is changed, specifically: 35, 51, 60, 77, 102, 120, 135, 157, 168, 185, 195, and 204 nm.

[0133] According to the experimental method of Example 1, the temperature of the central temperature zone of the tubular furnace and the placement position of the substrate are comprehensively controlled to synthesize CsPbBr3 nanosheets of different thicknesses. Nanosheets with a thickness range of 35 to 250 nm are selected, and CsPbBr3 / WS2 van der Waals heterojunctions are constructed respectively by dry transfer technology. Figure 3a shows a typical optical photograph of a van der Waals heterojunction formed by CsPbBr3 nanosheets of different thicknesses and the same WS2. Figure 3 b is the corresponding PL intensity mapping under 800nm ​​two-photon excitation, scale bar 10μm. Figure 3 c and Figure 3 d respectively show the corresponding Figure 3 The comparison of WS2PL spectra and lifetimes in different heterojunction regions in a shows that there are differences in the contribution of CsPbBr3 of different thicknesses to the enhancement of WS2 upconversion fluorescence and lifetime. Figure e shows the non-monotonic dependence of the WS2 upconversion fluorescence enhancement factor on the thickness of CsPbBr3, where the WS2 upconversion fluorescence enhancement factor = WS2 PL intensity in the heterojunction region / pure WS2PL intensity. Figure f shows a schematic diagram of step-type energy transfer in a heterojunction of CsPbBr3 nanosheets and WS2 with a certain thickness. Due to the deep penetration depth of two-photon excitation light, interference non-monotonic enhancement is formed inside the perovskite. Therefore, as the thickness of the perovskite increases, the deeper perovskite will be excited to produce stronger energy, which will promote the energy transfer of the upper perovskite to WS2. When the perovskite thickness increases to a certain extent (130-150nm), the deeper perovskite does not have enough light field to excite more energy, but will absorb the energy of the upper perovskite, thereby reducing the energy transferred to WS2. Therefore, as the thickness of the perovskite changes, the WS2 fluorescence enhancement shows a non-monotonic trend. When the CsPbBr3 thickness is in the range of 130 to 180 nm, and further in the range of 140 to 170 nm, it can show a better WS2 fluorescence enhancement advantage.

[0134] Example 5

[0135] Study on dark state energy transfer of “BAI / WS2” van der Waals heterojunction:

[0136] Compared with Example 2, the only difference is that the CsPbBr3 nanosheets are replaced with RP phase perovskite BAI. The two have matching band structures and can match the WS2 band to produce energy transfer.

[0137] The complete molecular formula of RP phase perovskite is C4H9NH3PbI4. In the present invention, a solution method is adopted to prepare its powder: PbO and C4H9NH3I source powders with a molar ratio of 1:1 are weighed; PbO is first added to a mixed solution of IH and H3PO2, and stirred on a magnetic heating stirring table at 60°C for about 4 hours until the mixed solution turns bright yellow; then C4H9NH3I source powder is added, and a precipitate is generated, and heating at 110°C and stirring for about 2 hours until the precipitate is completely dissolved; finally, the temperature is slowly lowered to obtain precipitated crystals of C4H9NH3PbI4, which are filtered, dried, and then dispersed on a tape for mechanical stripping.

[0138] like Figure 5 As shown in a, BAI perovskite nanosheets and single-layer WS2 were prepared by mechanical exfoliation, and then the van der Waals heterojunction was constructed using dry transfer technology. Subsequently, 800nm ​​two-photon excitation light was used to perform PL intensity mapping tests. The single-layer WS2 in the heterojunction area showed obvious fluorescence enhancement ( Figure 5 b), and the transient fluorescence test also shows that the BAI lifetime in the heterojunction region is shortened but the lifetime of the single-layer WS2 is prolonged ( Figure 5 e). This indicates that the dark state energy in BAI under two-photon excitation can also be transferred to WS2 to enhance its fluorescence emission. The method of enhancing TMDs fluorescence based on dark state energy transfer in the present invention has a certain universality.

[0139] Example 6

[0140] Research on “CsPbBr3 / WS2” van der Waals heterojunction near-infrared photodetector:

[0141] Figure 6 a shows an optical photograph of a typical CsPbBr3 / WS2 van der Waals heterojunction photodetector, using dry-transferred graphene as an electrode, and selecting the same pure single-layer WS2 as a comparison device, keeping the channel width of the heterojunction device consistent with that of the pure WS2 device. The inset is a schematic diagram of the photodetector. Figure 6 Figure b shows the comparison of the photocurrent data of the single-layer WS2 in the heterojunction region and the pure single-layer WS2 under 808nm two-photon excitation. It can be seen that the photocurrent in the heterojunction region is enhanced by an order of magnitude under the action of the dark state energy transfer mechanism.

[0142] Comparative Example 1

[0143] Compared with Example 2, the only difference is that single-photon excitation light is used for pumping.

[0144] The steady-state and transient tests of the van der Waals heterojunction in Example 2 both use 800nm ​​two-photon excitation light pumping, the steady-state test in Comparative Example 1 uses 488nm single-photon excitation light pumping, and the transient test uses 400nm single-photon excitation light pumping.

[0145] The difference lies in the carrier dynamics behavior at the heterojunction interface, such as Figure 7 a PL intensity mapping, Figure 7 PL spectra comparison of b and Figure 6 c shows that due to parity prohibition, the single-photon excitation light pump activates the bright exciton state of CsPbBr3 and the 1s bright state of WS2. Therefore, the carrier transfer is dominant at the heterojunction interface, resulting in double quenching of the fluorescence lifetime, as shown in Figure 7 c Inset shows the energy band diagram.

[0146] Comparative Example 2

[0147] Compared with Example 2, the only difference is that WS2 is replaced by WSe2 with mismatched energy band.

[0148] like Figure 8 As shown in a-8c, the fluorescence of heterojunction WSe2 shows quenching phenomenon under both single-photon excitation and two-photon excitation. Figure 8 The transient fluorescence spectrum of e-8h also shows similar behavior that the lifetimes of both CsPbBr3 and WSe2 in the heterojunction are shortened under single and double photon excitation, indicating that no energy transfer behavior occurs in this type II heterojunction. This is because the dark states (2p and 3p) in WSe2 do not form energy state overlap with the dark states of CsPbBr3.

[0149] Comparative Example 3

[0150] Compared with Example 6, the only difference is that single-photon excitation light is used for pumping.

[0151] like Fig. 9 As shown in b, under 450nm single-photon excitation light pumping, the photocurrent of the single-layer WS2 in the heterojunction region is compared with the photocurrent data of the pure single-layer WS2. It can be seen that the photocurrent in the heterojunction region is only enhanced by less than one time due to the bright-state carrier transfer mechanism.

Claims

1. A method for enhancing TMDs fluorescence based on a dark state energy transfer mechanism, characterized in that: A perovskite-TMDs heterojunction is obtained, which is then excited by a two-photon pump light source to accumulate dark state energy and induce parity-forbidden dark state energy transfer, thereby enhancing TMDs fluorescence. The perovskite-TMDs heterojunction comprises a perovskite nanosheet and TMDs compounded on the surface thereof, wherein the dark exciton state energy bands of the perovskite and TMDs match.

2. The method for enhancing TMDs fluorescence based on dark state energy transfer mechanism according to claim 1, characterized in that: The thickness of the perovskite nanosheet is below 250 nm, preferably 35 to 220 nm.

3. The method for enhancing TMDs fluorescence based on dark state energy transfer mechanism according to claim 1 or 2, characterized in that: The perovskite nanosheet is at least one of a CsPbX3 (X=Cl, Br, I) nanosheet or a two-dimensional RP phase perovskite; Preferably, the perovskite is CsPbBr3.

4. The method for enhancing TMDs fluorescence based on dark state energy transfer mechanism according to claim 3, characterized in that: Prepare the perovskite nanosheets by CVD method; Preferably, the CsPbBr3 nanosheets are prepared by a CVD method, the steps of which are: heating a mixed powder of CsBr and PbBr2 to volatilize, and chemically depositing CsPbBr3 nanosheets on the surface of a mica substrate with the assistance of a carrier gas; Preferably, the molar ratio of CsBr to PbBr2 is 1:1 to 1.1; Preferably, the heating temperature of the mixed powder is 550-580°C; Preferably, the mica substrate is located downstream of the mixed powder at a distance of 20 to 25 cm; Preferably, the carrier gas is at least one of the inert gases; Preferably, during the chemical deposition stage, the flow rate of the carrier gas is 10 to 100 sccm; Preferably, the chemical deposition time is 10 to 100 minutes.

5. The method for enhancing TMDs fluorescence based on dark state energy transfer mechanism according to claim 1, characterized in that: The TMDs are two-dimensional materials of at least one of sulfide and selenide of at least one element of W and Mo; Preferably, the TMDs is WS2.

6. The method for enhancing TMDs fluorescence based on dark state energy transfer mechanism according to claim 5, characterized in that: The TMDs are compounded on the surface of the perovskite nanosheets by mechanical exfoliation method and dry transfer technology; Preferably, TMDs can be peeled off on PMMA and then dry-transferred to perovskite nanosheets to obtain the perovskite-TMDs heterojunction.

7. The method for enhancing TMDs fluorescence based on dark state energy transfer mechanism according to claim 1, characterized in that: In the perovskite-TMDs heterojunction, an insulating spacer layer is compounded between the perovskite and the TMDs.

8. The method for enhancing TMDs fluorescence based on dark state energy transfer mechanism according to claim 7, characterized in that: The insulating spacer layer is a transparent material; further, hexagonal boron nitride (h-BN); Preferably, stacking and insulating spacer layers are formed on the surface of perovskite nanosheets by mechanical exfoliation method and dry transfer technique; Preferably, the thickness of the insulating spacer layer is less than 27 nm.

9. The method for enhancing TMDs fluorescence based on dark state energy transfer mechanism according to claim 1, characterized in that: The two-photon pumping light source is a femtosecond pulse laser with a wavelength of 800-1080nm.

10. A device, characterized in that: The method comprises the perovskite-TMDs heterojunction described in any one of claims 1 to 9 and performs TMDs fluorescence enhancement based on the method.

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