GB2595177A

GB2595177A - Method for characterizing defect in two-dimensional material and application thereof

Info

Publication number: GB2595177A
Application number: GB2111896.3A
Authority: GB
Inventors: Liu Dameng; Liu Huan; Wang Chong; Luo Jianbin
Original assignee: Tsinghua University
Current assignee: Tsinghua University
Priority date: 2019-01-31
Filing date: 2019-11-27
Publication date: 2021-11-17
Anticipated expiration: 2039-11-27
Also published as: GB2595177B; CN109765206A; WO2020155819A1; GB202111896D0; CN109765206B

Abstract

A method for characterizing a defect in a two-dimensional material and application thereof, relating to the technical field of the characterization of defects in nanomaterias. The method for characterizing a defect comprises: respectively and independently performing fluorescence lifetime imaging on a non-defective two-dimensional material substrate sample and a two-dimensional material substrate sample to be detected, and determining, according to a change of a fluorescence lifetime, whether there is a defect, wherein if the fluorescence lifetime of the two-dimensional material substrate sample to be detected is higher than the fluorescence lifetime of the non-defective two-dimensional material substrate sample, the two-dimensional material substrate sample to be detected is a defective sample, and if the fluorescence lifetime of the two-dimensional material substrate sample to be detected has no obvious change as compared with the fluorescence lifetime of the non-defective two-dimensional material substrate sample, the two-dimensional material substrate sample to be detected is a non-defective sample. The method characterizes a defect in a two-dimensional material using a fluorescence lifetime imaging method, can quickly and intuitively observe a change of a lifetime to determine whether the material is defective or not, and is a non-destructive detection method capable of implementing characterization at room temperature without introducing a new defect.

Description

METHOD FOR CHARACTERIZING DEFECTS IN TWO-DIMENSIONAL MATERIAL AND APPLICATIONS THEREOF

CROSS-REFERENCE TO RELATED APPLICATION

100011 The present application claims priority to Chinese patent application No 2019101014786 filed on January 31, 2019, entitled "Method for Characterizing Defects in Two-Dimensional Material and Applications Thereof', which is incorporated herein by reference in its entirety

FIELD OF TECHNOLOGY

100021 The present application relates to the technical field of characterization of defects in nano-materials, and in particular to a method for characterizing defects in a two-dimensional material and an application thereof.

BACKGROUND

100031 Two-dimensional materials have been widely applied due to their many unique electrical, optical, chemical and thermal properties. For example, they play a very important role in the construction of microelectronics and optoelectronic components, semiconductor devices, and solar cells. Meanwhile, since two-dimensional materials are suitable as a carrier for studying the structure and physical properties of materials, and may also be used as basic structural units for constructing other dimensional materials, the research on two-dimensional materials is very important.

100041 However, it is difficult for two-dimensional materials to exist in large quantities in nature. Generally they are peeled from natural materials by artificial means or synthesized by other substances. Two-dimensional materials made by various methods inevitably have certain defects which will seriously affect the performance of an element. Therefore, means for characterization and identification of defects is particularly important.

100051 At present, there are mainly two common characterization means: transmission electron microscopy (TEM) and spectroscopic characterization. The resolution of TEM is generally at a nanometer level, and the atomic structure may be observed. Generally, atomic images may be observed after the surface lattice images are filtered, and thus the defects may be seen from high-resolution TENI images (as shown in FIG. 1). Spectroscopic characterization mainly includes Raman and fluorescent spectroscopies. For example, when Raman spectroscopy is used to identify grapheme, defects in graphene cause the Raman spectrum to exhibit two new vibration modes, namely D peak (1350cm-') and D Peak (1620cm-') (as shown in FIG. 2). For fluorescent spectroscopy, defects may cause fluorescence peaks in two-dimensional materials, and thus the defects may be analyzed according to the positions of the fluorescence peaks.

[0006] However, these methods have the following shortcomings. The electron beam used in the TEM characterization process is relatively high, which causes new defects. In addition, the TENI characterization process has such shortcomings as relatively small characterization area, strict requirements for preparation of samples, high cost and low efficiency The time being taken to characterize defective samples using Raman spectroscopy is long, the laser spot for Raman characterization is in the order of micrometer, the efficiency is low, and it is impossible to characterize in a large area by Raman scanning. When using fluorescence spectrum to characterize two-dimensional materials, most of the time, it needs to be performed at low temperature (the low temperature is sensitive to defects).

[0007] Therefore, it is desirable to provide a novel method for characterizing defects of a two-dimensional material, which may solve at least one of the above-mentioned problems [0008] As a result, the present application is proposed. BRIEF SUMMARY [0009] A first objective of the present application is to provide a method for characterizing defects in a two-dimensional material, which is a quick, intuitive, and non-destructive method capable of implementing characterization at room temperature by fluorescence lifetime imaging method.

[OM] A second objective of the present application is to provide an application of a method for characterizing defects in a two-dimensional material in the detection of elements based on the two-dimensional material.

[0011] In order to achieve the above-mentioned objectives of the present application, the following technical solutions are specially adopted 100121 In a first aspect, a method for characterizing defects in a two-dimensional material is provided, which includes the following steps: [0013] (a) providing a defect-free two-dimensional material substrate sample and a two-dimensional material substrate sample to be detected, and [0014] (b) under a same excitation wavelength, respectively and independently performing fluorescence lifetime imaging on the defect-free two-dimensional material substrate sample and the two-dimensional material substrate sample to be detected, and determining whether there is a defect according to a change in a fluorescence lifetime, wherein when the fluorescence lifetime of the two-dimensional material substrate sample to be detected is higher than that of the defect-free two-dimensional material substrate sample, the two-dimensional material substrate sample to be detected is a defective sample; and when the fluorescence lifetime of the two-dimensional material substrate sample to be detected has no significant change as compared with that of the defect-free two-dimensional material substrate sample, the two-dimensional material substrate sample to be detected is a defect-free sample [0015] In an embodiment, on the basis of the technical solution according to the present application, the fluorescence lifetime in step (b) is obtained through a fluorescence lifetime image or a fluorescence lifetime decay curve.

[0016] In an embodiment, on the basis of the technical solution according to the present application, step (b) further includes judging the number of defects according to the degree of change in the fluorescence lifetime: the greater the difference between the fluorescence lifetime of the two-dimensional material substrate sample to be detected and that of the defect-free two-dimensional material substrate sample, the more the number of defects in the samples.

[0017] In an embodiment, on the basis of the technical solution according to the present application, in step (b), a fluorescence lifetime imaging system is used in fluorescence lifetime imaging on samples. Performing fluorescence lifetime imaging on samples by the fluorescence lifetime imaging system includes the following steps: 100181 emitting laser light by a laser, allowing the laser light to pass through a galvanometer, then reflecting the laser light by a beam splitter to objective lens to be focused on the sample, collecting optical signal generated by the sample through the objective lens and transmitting it through the beam splitter, and detecting a fluorescence lifetime of the sample using a filter; then detecting the optical signal using a photoelectric detector, synchronizing the photoelectric detector and the laser using a time-correlated single photon counting system, and obtaining a fluorescence lifetime image by scanning using the galvanometer.

[0019] In an embodiment, on the basis of the technical solution according to the present application, the excitation wavelength of the laser is 450-500 nm, and the excitation frequency of the laser is 35-45 MHz; [0020] in an embodiment, the wavelength of the filter is 500-700 nm; and [0021] in an embodiment, a resolution of the time-dependent single photon counting system is 6-10 ps.

[0022] In an embodiment, on the basis of the technical solution according to the present application, the two-dimensional material includes a two-dimensional material directly grown on a substrate by chemical vapor deposition, or a two-dimensional material transferred to the substrate by a mechanical lift-off or photoresist transfer method.

[0023] In an embodiment, the two-dimensional material includes transition metal sulfide, transition metal selenide or transition metal telluride, preferably one of WS2, MoS2, ReS,), WSe2, MoSe,), Bi2Se3, MoTe2, WTe2 and Bi2Te3 [0024] In an embodiment, on the basis of the technical solution according to the present application, the substrate includes a metal copper, nickel, platinum, iron or alloy substrate [0025] In an embodiment, on the basis of the technical solution according to the present application, the defects include one or more of point defects, grain boundary defects, wrinkles and broken edges [0026] In a second aspect, an application of a method for characterizing defects in a two-dimensional material in the detection of elements based on the two-dimensional material is provided [0027] In an embodiment, the elements based on the two-dimensional material include a diode, a spin element, a field effect transistor or a tunneling transistor.

[0028] Compared with the prior art, the present application has the following beneficial effects: [0029] the method characterizes a defect in a two-dimensional material using a fluorescence lifetime imaging method, which may quickly and intuitively observe a change in the fluorescence lifetime so as to determine whether the material is defective or not, and is a non-destructive detection method capable of implementing characterization at room temperature due to insensitivity of the fluorescence lifetime imaging method to the temperature, without introducing a new defect and with large characterization area, fast imaging and high efficiency

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application, and therefore should not be regarded as a limitation to the scope. Other drawings may be obtained according to these drawings without any creative work for those skilled in the art.

[0031] FIG. 1 is a schematic diagram of a method for characterizing defects of a two-dimensional material using TEM in the prior art; [0032] FIG. 2 is a schematic diagram of a method for characterizing defects of a two-dimensional material using spectroscopy in the prior art (wherein (a) is a schematic diagram of a method for characterizing defects in graphene using Raman spectroscopy, and (b) is a schematic diagram of the fluorescence peaks generated by characterizing defects in WSe2 using fluorescence spectroscopy); [0033] FIG. 3 is a schematic structural diagram of a fluorescence lifetime imaging system according to an embodiment of the present application; [0034] FIG. 4 shows the fluorescence lifetime images of a WS9 sample before and after plasma treatment in Embodiment 1 at an excitation wavelength of 561 nm (where (a) is the fluorescence lifetime image of the WS2 sample before plasma treatment at the excitation wavelength of 561 nm, and (b) is the fluorescence lifetime image of the WS2 sample after plasma treatment at the excitation wavelength of 561 nm); [0035] FIG. 5 is fluorescence spectra of an original mono-layer WS2 before and after plasma treatment; [0036] FIG. 6 is a time-resolved fluorescence lifetime decay curve of original and defective WS? samples (where (a) is the time-resolved fluorescence lifetime decay curve of the original WS2 sample, and (b) is the time-resolved fluorescence lifetime decay curve of the defective WS? sample); [0037] FIG. 7 shows the effect of different excitons on exciton-exciton annihilation (where (a) is a graph of the relationship between the fluorescence spectrum weight of neutral excitons, trion excitons and defect-state excitons and laser power, (b) shows time-resolved fluorescence lifetime decay curves fitted by a double exponential function of the neutral exciton and trion exciton lifetimes in the original and defective mono-layer WS?, (c) shows time-resolved fluorescence lifetime decay curves fitted by a double exponential function of the lifetime weights of the neutral exciton and trion exciton in the original and defective mono-layer WS2, (d) is a diagram showing average lifetimes of the original and defective WS2 under different excitation intensities, and (e) is a graph of the relationship between the steady-state fluorescence intensity of neutral excitons in the original and defective mono-layer WS2 and the laser power; [0038] FIG. 8 shows fluorescence lifetime imaging images of neutral excitons and defect-state excitons under different excitation intensities (where (a) shows fluorescence lifetime imaging images of the neutral excitons in the original mono-layer WS2 under the different excitation intensities, and (b) shows fluorescence lifetime imaging images of the neutral excitons in the defective mono-layer WS2 under different the excitation intensities and (c) shows fluorescence lifetime imaging images of the defect-state excitons in the defective mono-layer WS2 under the different excitation intensities); [0039] FIG. 9 is a time-resolved fluorescence lifetime decay curve of neutral excitons in the original mono-layer WS2 under different excitation intensities; [0040] FIG. 10 is a time-resolved fluorescence lifetime decay curve of neutral excitons in the defective mono-layer WS2 under different excitation intensities; 100411 FIG. 11 is a time-resolved fluorescence lifetime decay curve of defect-state excitons in the defective mono-layer WS2 under different excitation intensities; [0042] FIG. 12 is a linear graph obtained by fitting a time-resolved fluorescence lifetime decay curve of neutral excitons in the original mono-layer WS? under different excitation intensities; [0043] FIG. 13 is a linear graph obtained by fitting a time-resolved fluorescence lifetime decay curve of neutral excitons in the defective mono-layer WS2 under different excitation intensities; [0044] FIG. 14 is a linear graph obtained by fitting a time-resolved fluorescence lifetime decay curve of defect-state excitons in the defective mono-layer WS2 under different excitation intensities, and [0045] FIG. 15 is a diagram showing the exciton dynamics and EEA processes in the original mono-layer WS2 and defective mono-layer WS, [0046] In the drawings 1-laser driver, 2-laser head; 3-galvanometer, 4-beam splitter, 5-objective lens, 6-sample, 7-filter, 8-photoelectric detector, 9-TCSPC.

DETAILED DESCRIPTION

[0047] The implementations of the present application will be described in detail below in conjunction with embodiments, but those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. When specific conditions are not indicated, the embodiments shall be carried out in accordance with conventional conditions or conditions recommended by the manufacturer. The used reagents or instruments without indication of manufacturer are all conventional products that may be commercially available.

[0048] According to the first aspect of the present application, a method for characterizing defects in a two-dimensional material is provided, including the following steps: [0049] (a) providing a defect-free two-dimensional material substrate sample and a two-dimensional material substrate sample to be detected; and [0050] (b) under a same excitation wavelength, respectively and independently performing fluorescence lifetime imaging on the defect-free two-dimensional material substrate sample and the two-dimensional material substrate sample to be detected, and determining whether there is a defect according to a change in a fluorescence lifetime: if the fluorescence lifetime of the two-dimensional material substrate sample to be detected is higher than that of the defect-free two-dimensional material substrate sample, the two-dimensional material substrate sample to be detected is a defective sample, and if the fluorescence lifetime of the two-dimensional material substrate sample to be detected has no significant change as compared with that of the defect-free two-dimensional material substrate sample, the two-dimensional material substrate sample to be detected is a defect-free sample.

100511 Two-dimensional materials refer to materials in which electrons may only move freely in two dimensions (planar motion) of non-nanoscale (1-100 nm).

100521 It may be understood that the two-dimensional material here refer to a two-dimensional material capable of emitting fluorescence. Typical but non-limiting examples are transition metal sulfides, transition metal selenides, transition metal tellurides or the like, including but not limited to one of WS2, MoS2, ReS2, WSe2, MoSe2, Bi2Se3, MoTe2, WTe2 and Bi2Te3. As a typical exemplary solution, the two-dimensional material is tungsten disulfide (WS?).

[0053] The source of the two-dimensional material is not limited, which may be a two-dimensional material directly grown on the substrate by chemical vapor deposition, or may also be a two-dimensional material transferred to the substrate by a mechanical lift-off or photoresist transfer method.

[0054] Mechanical lift-off is to peel a single layer of two-dimensional material from the surface of the two-dimensional material crystal using a mechanical force, that is, a layer of two-dimensional material may be directly peeled off from the two-dimensional material with tape, and then repeatedly pasted between the tapes to make the layer of the two-dimensional material thinner and thinner, and then the tape is attached to the substrate, so that the mono-layer two-dimensional material is transferred to the substrate.

[0055] The types of defects are not limited, including but not limited to grain boundary defects, wrinkles, broken edges, point defects or the like [0056] The substrate is not limited, and may be but not limited to, a metal copper, nickel, platinum, iron or alloy substrate, and may be a base material or a flexible base used in a semiconductor manufacturing process. It may be determined according to the final element to be prepared.

[0057] A defect-free two-dimensional material substrate sample refers to a substrate with a perfect two-dimensional material, that is, the two-dimensional material on the substrate sample is a perfect two-dimensional material without defects.

[0058] In the present application, a defect-free two-dimensional material substrate sample is used as a reference sample, and the fluorescence lifetime of the reference sample at a certain excitation wavelength is detected by a fluorescence lifetime imaging method and taken as a standard.

[0059] When the sample to be detected is detected, the two-dimensional material substrate sample to be detected is subjected to fluorescence lifetime imaging under the same condition (the same excitation wavelength), and it is determined whether there is a defect by comparing with the fluorescence lifetime of the reference sample.

[0060] If the fluorescence lifetime of the two-dimensional material substrate sample to be detected is higher than that of the reference sample, the two-dimensional material substrate sample to be detected is a defective sample; and if the fluorescence lifetime of the two-dimensional material substrate sample to be detected has no significant change as compared with that of the reference sample, the two-dimensional material substrate sample to be detected is a defect-free sample.

[0061] Fluorescence lifetime refers to an average time at which fluorescence stays in the excited state, and is about the order of ns. Fluorescence lifetime is a characteristic of the molecule itself and is independent of the concentration of a fluorophore and the intensity of the excitation light 100621 Fluorescence lifetime imaging (fluorescence lifetime microscopic imaging) technology is to study the lifetime of fluorescence using a combination of microscope and lifetime measurement technology. Fluorescence lifetime measurement methods include frequency domain method and time domain method. Time domain method is preferred. Time domain method is also referred to as pulse method, which excites a fluorescent sample using ultrashort pulse laser and then measures an intensity decay curve of fluorescence of the sample to calculate the fluorescence lifetime. A typical but non-limiting method is a time-correlated single photon counting (TCSPC). The working principle of the lifetime measurement based on the TCSPC method is as follows. A sample is excited with high repetitive pulse excitation light. In each pulse cycle, a fluorescent molecule is excited to emit one photon at most, so that at most one photon may be detected in each cycle, and then the timing at which the photon appears is recorded, and one photon is recorded at that timing. The same happens in the next pulse cycle. A distribution curve of fluorescence photons over time, which is equivalent to the fluorescence decay curve, may be obtained after counting many times, and finally the fluorescence lifetime of the sample may can be obtained by fitting the decay curve or other forms of data analysis.

[0063] Two types of data may be obtained through fluorescence lifetime imaging test: fluorescence lifetime image or fluorescence lifetime decay curve. The fluorescence lifetime image may visually indicate the fluorescence lifetime, and the fluorescence lifetime decay curve may be used to obtain the fluorescence lifetime of the sample through fitting or other forms of data analysis.

[0064] The different colors in the fluorescence lifetime image represent the length of the fluorescence lifetime, and the change in the fluorescence lifetime of a material may be visually observed. That is, the fluorescence lifetime may be directly judged according to the respective fluorescence lifetime imaging graphs of the reference sample and the two-dimensional material substrate sample to be detected, and the sample with a relatively long fluorescence lifetime is a defective two-dimensional material substrate sample.

[0065] By enlarging a certain area on the fluorescence lifetime image, the fluorescence lifetime decay curve of this area is obtained, and then the fluorescence lifetime of this area may be obtained by fitting or other forms of data analysis.

100661 It should be noted that the average value of the fluorescence lifetime of each area is calculated as the fluorescence lifetime of the entire sample. Therefore, if the average fluorescence lifetime of the two-dimensional material substrate sample to be detected is higher than that of the reference sample, the two-dimensional material substrate sample to be detected is a defective sample; and when the average fluorescence lifetime of the two-dimensional material substrate sample to be detected has no significant change as compared with that of the reference sample, the two-dimensional material substrate sample to be detected is a defect-free sample.

[0067] No significant change means that the fluorescence lifetime fluctuates within ±5%.

[0068] Light excitation makes the electron jump from a ground state to an excited state, and the electron is subjected to relaxation and recombines with the hole to emit photons. When a defect exists, a defect fluorescence peak may be generated The defect fluorescence lifetime is generally longer than the inherent fluorescence lifetime, causing the average fluorescence lifetime of the defective sample to become longer.

[0069] The present application characterizes defects in a two-dimensional material using a fluorescence lifetime imaging method, which judges whether the material is defective or not according to a change in the fluorescence lifetime and may detect defects more quickly and intuitively by characterizing the effect of the defects on the fluorescence lifetime through the imaging method. In addition, the characterization of the fluorescence lifetime may be implemented at room temperature due to insensitivity of the imaging method to the temperature without introducing a new defect, and this method is a non-destructive detection method having large characterization area, fast imaging and high efficiency.

[0070] In one embodiment, step (b) further includes judging the number of defects according to the degree of change in the fluorescence lifetime. The degree of change in the fluorescence lifetime refers to the difference between the fluorescence lifetime of the sample to be detected and that of the reference sample, that is, the greater the difference between the fluorescence lifetime of the two-dimensional material substrate sample to be detected and that of the defect-free two-dimensional material substrate sample, the more the number of defects in the sample to be detected.

100711 The length of the fluorescence lifetime is negatively correlated with the number of defects in the sample, and the decay rate is positively correlated with the number of defects in the sample, that is, the shorter the fluorescence lifetime, the faster the decay rate, and the more defects introduced in the two-dimensional material substrate sample. Based on this, the number of defects may be roughly judged [0072] In one embodiment, a TCSPC FLIM system generally includes: a confocal microscopy imaging system, a TCSPC counter, a FLINT detector, and an analysis software.

[0073] In an embodiment, a typical FLINT system is shown in FIG. 3, including a laser (a laser driver 1, a laser head 2), a galvanometer 3, a beam splitter 4, an objective lens 5, a sample 6, a filter 7, a photoelectric detector 8, and TCSPC 9. The fluorescence lifetime imaging process performed through this system is as follows: [0074] emitting laser light by the laser, allowing the laser light to pass through the galvanometer 3, then reflecting the laser light by the beam splitter 4 to the objective lens 5 to be focused on the sample 6, collecting optical signal generated by the sample through the objective lens 5 and transmitting it through the beam splitter 4, and detecting a fluorescence lifetime of the sample using the filter 7; then detecting the optical signal using the photoelectric detector 8, synchronizing the photoelectric detector and the laser using the TCSPC 9, and obtaining a fluorescence lifetime image by scanning using the galvanometer.

[0075] No limitations are made to the wavelength frequency of the laser, the wavelength of the filter, and the resolution of the single photon counting system.

[0076] In an embodiment, the excitation wavelength of the laser is 450-500 nm, such as 450 nm, 460 nm, 470 nm, 488 nm, 495 nm or 500 nm, and the excitation frequency is 35-45 MHz, such as 35 MHz, 36 MHz, 37 MHz, 38 MHz, 39 MHz, 40 MHz, 41 MHz, 42 MHz, 43 MHz, 44 MHz, or 45 MHz.

[0077] In an embodiment, the wavelength of the filter is 500-700 nm, such as 500 nm, 550 nm, 561 nm, 600 nm, 624 nm, 650 nm or 700 nm 100781 In an embodiment, the resolution of a time-dependent single photon counting system is 6-10 ps, for example, 6 ps, 7 ps, 8 ps, 9 ps or 10 ps.

100791 In this system, optimizing the parameters may provide better imaging effect.

100801 According to the second aspect of the present application, an application of the above method for characterizing defects in a two-dimensional material in the detection of elements based on the two-dimensional material is provided.

100811 In an embodiment, the elements based on the two-dimensional material include a diode, a spin element, a field effect transistor or a tunneling transistor.

[0082] The diode is an electronic element with two electrodes that only allows current to flow in a single direction, a spin element is an electronic element with spin properties; a field-effect transistor is a semiconductor device that controls currents in an output circuit by controlling the electric field effect of an input circuit, and the tunneling transistor is a crystal device with a tunneling effect.

100831 The method for characterizing defects in a two-dimensional material of the present application may be used to detect whether a sample based on two-dimensional material is defective or not. Since defects in the two-dimensional material may affect the performance of elements, it is of great significance to detect whether the two-dimensional material is defective. The method of the present application may quickly pick out defective elements based on two-dimensional materials, avoiding that the performance of the elements is determined using time-consuming electrical measurement methods.

100841 In order to further understand the present application, the methods and effects of the present application will be further described in detail below in conjunction with specific embodiments These embodiments are only typical descriptions of the present application, but the present application is not limited thereto. Unless otherwise specified, the test methods used in the following embodiments are conventional methods, and the raw materials and reagents used are all those that may be commercially available from conventional commercial channels.

100851 Embodiment 1 Detection of defects in WS2 by fluorescence lifetime imaging method [0086] The two-dimensional material WS) was prepared on an exfoliation substrate by the mechanical lift-off method [0087] Preparation of a defective sample: defects were introduced into the sample using a plasma cleaner having a power of 20W, and a radio frequency of 13.56 NTH, and then the sample was bombarded with argon for 10 seconds.

[0088] Fluorescence lifetime imaging detection was performed on the defective sample using a fluorescence lifetime imaging system (PicoHarp 300, PicoQuant), as shown in FIG. 3. The excitation wavelength is 488 nm, the frequency is 40 MHz, and the objective lens (40X, NA 0.95) was used to focus the laser for exciting the sample, the generated fluorescence signal was collected by the same objective lens, a 561 nm long-pass filter was used to filter the fluorescence lifetimes of the original and defective mono-layer WS2, the filtered optical signal was detected by a photoelectric detector, and then the photoelectric detector and the laser were synchronized using a time-correlated single photon counting system (TSSPC) to obtain the fluorescence lifetime of each spot position, and finally the fluorescence lifetime images were obtained through the galvanometer scanning. The resolution of the TCSPC was 8.0ps.

[0089] The fluorescence lifetime images of the original (before plasma treatment) WS2 sample and the defective (after plasma treatment) WS2 sample at the excitation wavelength of 561 nm are shown in FIG. 4 [0090] It can be seen intuitively from FIG. 4 that different fluorescence lifetimes are represented by different colors, and the fluorescence lifetime of the defective sample is significantly longer.

[0091] Embodiment 2 Confirming that defects make the fluorescence lifetime longer [0092] The fluorescence spectrum of the original mono-layer WS) is shown in FIG 5 It can be seen that the fluorescence peak of WS) before plasma treatment is formed by overlapping a neutral exciton peak with a trion peak. After plasma treatment, a new peak, namely the defect-state exciton peak appears.

[0093] The time-resolved fluorescence lifetime decay curves of the original and defective samples are shown in FIG. 6. It can be seen from FIG. 6 that the decay rate of the defect-state excitons is significantly reduced, and the fluorescence lifetime thereof is significantly longer, indicating that the fluorescence lifetime is lengthened due to the defects.

[0094] It can be seen that light excitation makes the electron jump from a ground state to an excited state, and the electron is subjected to relaxation and recombines with the hole to emit photons. When a defect exists, a defect fluorescence peak may be generated. The defect fluorescence lifetime is generally longer than the inherent fluorescence lifetime, causing the average fluorescence lifetime of the defective sample to become longer.

[0095] Embodiment 3 Effects of defects on the exciton-exciton annihilation process in a mono-layer tungsten disulfide [0096] The existence of defects will also cause the rate of exciton-exciton annihilation (EEA) to decrease Exciton-exciton annihilation is a process in which an exciton transfers energy to another exciton and belongs to a non-radiative process, so that it may shorten the fluorescence lifetime, and the exciton annihilation rate may be obtained through data processing When the defect exists, intrinsic excitons in the sample may be bound to form defect-state excitons, which will decrease the number of intrinsic excitons, so that the number of excitons used to participate in the exciton annihilation process is decreased, and the annihilation rate is reduced.

[0097] FIG. 7 shows effects of different excitons on exciton-exciton annihilation. FIG. 7(a) shows fluorescence spectrum weights of neutral excitons, trion excitons and defect-state excitons as a function of laser power. All fluorescence spectrum values are all normalized by the peak value. It is found by the study that a contribution rate of the neutral exciton peak decreases, and the contribution rates of others are opposite to the decreasing trend of the contribution rate of the neutral exciton peak. This is because the decrease in the neutral exciton peak leads to an increase in the trion exciton and defect-state exciton peaks. At the same time, the trion excitons have no significant contribution to the defect-state excitons, indicating that only the neutral excitons are bound by the defects. The effect of trion excitons on exciton-exciton annihilation is ignored herein. This may also be confirmed by measuring the time-resolved fluorescence lifetime decay curves under different excitation intensities. With the increase of excitation intensity, the lifetime weight of the neutral excitons is greater than 99%, whether it is the original or defective mono-layer, which contributes more to the time-resolved fluorescence lifetime decay curves than the lifetime weight (less than 1%) of the trion excitons in FIG. 7 (c). For exciton lifetime, unlike the monotonic decrease of neutral excitons, the lifetime of trion excitons fluctuates in FIG. 7(b), so that the effect of trion excitons on EEA is ignored.

[0098] At the same time, the average lifetimes of the original sample and the defective sample in FIG. 7(d) decrease with the increase in excitation intensity, indicating that the nonlinear attenuation channel EEA becomes the dominant relaxation channel under high excitation intensity and competes with a radiative recombination channel, leading to shortening of exciton lifetime. In addition, the fluorescence lifetime of the defective sample is longer than that of the original sample, indicating that the defects cause the reduction of EEA to some extent. In FIG. 7(e), the dependence of the peak intensity of the neutral exciton on the laser power is plotted. At low laser power, the integrated fluorescence intensities of the original mono-layer and the defective mono-layer are closed, indicating that the defects have small effects on dynamics of neutral excitons. However, at high laser power, EEA becomes very significant due to the quantum confinement effect and the strong Coulomb interaction in the mono-layer tungsten disulfide. The fluorescence intensity of the original mono-layer WS2 under high laser power is higher than that of the defective sample, indicating that the defects may bind the neutral excitons and have a significant effect on the EEA process of the defective mono-layer WS2.

[0099] FIG. 8 shows fluorescence lifetime imaging images of neutral and defect-state excitons under different excitation intensities. As shown in FIG. 8(a), (b), (c), it was found that the exciton lifetime under high excitation intensity is shorter than that under low excitation intensity. The same result is also observed for defect-state excitons in FIG. 8(c), which indicates that EEA occurs in mono-layer WS2 under high excitation intensity In addition, the time-resolved fluorescence lifetime decay curves (FIGS. 9 to 11) under different excitation intensities were normalized to obtain FIGS. 12 to 14, indicating that the fluorescence lifetime dynamics strongly depends on excitation intensities corresponding to different initial excitation densities n(0). FIGS. 12 to 14 show data obtained by linearizing TRPL curves of neutral excitons and defect-state excitons using equation (1), and the solid line shows a linear fit.

no exp(-1,c,t) [00100] n(0- (1) 1+ (1(0)7n, p _ exp(-A-001 [OMN] Where n(t) represents exciton groups, no is an initial exciton density, and t is the decay time. 1(0 =1/1-0 is an internal exciton recombination rate, and To is the PL lifetime or defect binding of neutral excitons at low excitation intensity without exciton-exciton annihilation. 7 is the exciton annihilation rate constant, assuming that it is independent of the decay time.

[00102] For mono-layer WS2, the exciton density was estimated using an absorption coefficient of 3.5%, and the energy of each pulse was calculated. It is assumed that the initial exciton density is related to neutral excitons, and each photon may excite one exciton. As the density increases, the decay trend develops rapidly, wherein the decay signal of the initial exciton density below has relatively slow relaxation kinetics. According to previous studies, when the exciton density exceeds 1010 cm', the EEA in the mono-layer WS2 may be triggered. As the density continues to increase, exciton-exciton annihilation (EEA) controls the exciton dynamics leading to faster decay, which is an additional important non-radiative relaxation channel.

[00103] Possible exciton dynamics and EEA process in the single layers of original and defective tungsten disulfide are shown in FIG. 15: when the sample is excited at a low excitation intensity, the main exciton relaxation channel is of radiative recombination of excitons, and EEA may be ignored. Under the high excitation intensity of the original sample, EEA becomes vent significant, and is a scattering process in which energy transfers from one exciton to another exciton, and then is excited to a high-energy state, and then is relaxed to a low energy state through electron-phonon interaction, the excited state is relaxed to a ground state through a non-radiative relaxation pathway. As the density increases and the excitation intensity increases, the PL decay rate increases, and the excited state decay rate is accelerated. After the defects are introduced, some excited neutral excitons are trapped by the defects. Because the excitons may be bound by the introduced defects, the number of excitons used to generate EEA is decreased, causing the reduction in the EEA rate. This situation is similar to self-trapping of excitons in one-dimensional organic metal halide nanotubes. In addition, the EEA rate is related to exciton diffusion, and defects may inhibit the exciton diffusion. In addition, the defect-state excitons also promote radiative relaxation and become an energy dissipation path that competes with the exciton annihilation process.

[00104] In the defective monomolecular layer of tungsten disulfide, the EEA rate of defect-state excitons is lower than that of neutral excitons. Due to the low quantum efficiency of PL, the number of defect-state excitons is less than that of neutral excitons. On the other hand, the exciton binding energy of the defect-state excitons is less than that of the neutral excitons in the mono-layer, which leads to more nonlocality and faster exciton diffusion. The exciton diffusion constant is proportional to the diffusion length. The longer the diffusion length of defect-state excitons, the longer the time of EEA when two excitons are close to each other.

[00105] Although specific embodiments have been used to illustrate and describe the present application, it should be appreciated that many other changes and modifications may be made without departing from the spirit and scope of the present application. Therefore, this means that all these changes and modifications that fall within the scope of the present application are included in the appended claims.

Claims

Claims: 1. A method for characterizing defects in a two-dimensional material, comprising the following steps: (a) providing a defect-free two-dimensional material substrate sample and a two-dimensional material substrate sample to be detected; and (b) under a same excitation wavelength, respectively and independently performing fluorescence lifetime imaging on the defect-free two-dimensional material substrate sample and the two-dimensional material substrate sample to be detected, and determining whether there is a defect according to a change in a fluorescence lifetime: wherein when the fluorescence lifetime of the two-dimensional material substrate sample to be detected is higher than that of the defect-free two-dimensional material substrate sample, the two-dimensional material substrate sample to be detected is a defective sample, and when the fluorescence lifetime of the two-dimensional material substrate sample to be detected has no significant change as compared with that of the defect-free two-dimensional material substrate sample, the two-dimensional material substrate sample to be detected is a defect-free sample.
2 The method for characterizing defects in a two-dimensional material of claim 1, characterized in that the fluorescence lifetime in step (b) is obtained through a fluorescence lifetime image or a fluorescence lifetime decay curve.
3. The method for characterizing defects in a two-dimensional material of claim I, characterized in that step (b) further comprises judging the number of defects according to the degree of change in the fluorescence lifetime: the greater the difference between the fluorescence lifetime of the two-dimensional material substrate sample to be detected and that of the defect-free two-dimensional material substrate sample, the more the number of defects in the sample to be detected.
4. The method for characterizing defects in a two-dimensional material of any one of claims 1 to 3, characterized in that in step (b), the performing fluorescence lifetime imaging on the sample uses a fluorescence lifetime imaging system and comprises the following steps: emitting laser light by a laser, allowing the laser light to pass through a galvanometer, then reflecting the laser light by a beam splitter to objective lens to be focused on the sample, collecting optical signal generated by the sample through the objective lens and transmitting it through the beam splitter, detecting a fluorescence lifetime of the sample using a filter; then detecting the optical signal using a photoelectric detector, synchronizing the photoelectric detector and the laser using a time-correlated single photon counting system, and obtaining a fluorescence lifetime image by scanning using the galvanometer.
5. The method for characterizing defects in a two-dimensional material of claim 4, characterized in that the excitation wavelength of the laser is 450-500 nm, and the excitation frequency of the laser is 35-45 I\THz; the wavelength of the filter is 500-700 nm, and a resolution of the time-dependent single photon counting system is 6-10 ps.
6. The method for characterizing defects in a two-dimensional material of any one of claims 1 to 3, characterized in that the two-dimensional material comprises a two-dimensional material directly grown on a substrate by chemical vapor deposition, or a two-dimensional material transferred to the substrate by a mechanical lift-off or photoresist transfer method; preferably, the two-dimensional material comprises transition metal sulfide, transition metal selenide or transition metal telluride, preferably one of WS2, MoS,, ReS2, WSe2, MoSe2, Bi2Se3, MoTe2, WTe2 and Bi2Te3.
7. The method for characterizing defects in a two-dimensional material of any one of claims Ito 3, characterized in that the substrate comprises a metal copper, nickel, platinum, iron or alloy substrate.
8. The method for characterizing defects in a two-dimensional material of any one of claims 1 to 3, characterized in that the defects comprise one or more of point defects, grain boundary defects, wrinkles and broken edges.
9. An application of the method for characterizing defects in a two-dimensional material according to any one of claims 1 to 8 in the detection of elements based on the two-dimensional material
10. The application of claim 9, characterized in that the elements based on the two-dimensional material comprise a diode, a spin element, a field effect transistor or a tunneling transistor.