WO2020155819A1 - 表征二维材料缺陷的方法及其应用 - Google Patents

表征二维材料缺陷的方法及其应用 Download PDF

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WO2020155819A1
WO2020155819A1 PCT/CN2019/121186 CN2019121186W WO2020155819A1 WO 2020155819 A1 WO2020155819 A1 WO 2020155819A1 CN 2019121186 W CN2019121186 W CN 2019121186W WO 2020155819 A1 WO2020155819 A1 WO 2020155819A1
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dimensional material
fluorescence lifetime
sample
defects
material substrate
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刘大猛
刘欢
王冲
雒建斌
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Tsinghua University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/64—Fluorescence; Phosphorescence
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/64—Fluorescence; Phosphorescence
    • G01N21/6408—Fluorescence; Phosphorescence with measurement of decay time, time resolved fluorescence
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/64—Fluorescence; Phosphorescence
    • G01N21/645—Specially adapted constructive features of fluorimeters
    • G01N21/6456—Spatial resolved fluorescence measurements; Imaging
    • G01N21/6458—Fluorescence microscopy
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/64—Fluorescence; Phosphorescence
    • G01N21/645—Specially adapted constructive features of fluorimeters
    • G01N21/6456—Spatial resolved fluorescence measurements; Imaging
    • G01N2021/646—Detecting fluorescent inhomogeneities at a position, e.g. for detecting defects

Definitions

  • This application relates to the technical field of nanomaterial defect characterization, and specifically, to a method for characterizing two-dimensional material defects and its application.
  • Two-dimensional materials have many unique electrical, optical, chemical and thermal properties, making two-dimensional materials widely used, for example, in the construction of microelectronics and optoelectronic components, semiconductor devices and solar cells play a very important role.
  • two-dimensional materials are suitable as a carrier for studying the structure and physical properties of materials, and can also be used as basic structural units for constructing other dimensional materials, because the research on two-dimensional materials is very important.
  • two-dimensional materials 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 materials. However, two-dimensional materials made by various methods inevitably have certain defects. These defects will seriously affect the performance of the component. Therefore, the characterization and identification of defects is particularly important.
  • TEM transmission electron microscopy
  • spectroscopy mainly includes Raman and fluorescence spectroscopy.
  • Raman spectroscopy when using Raman spectroscopy to identify graphene, the defects in graphene cause two new vibration modes in the Raman spectrum, namely D peak (1350cm -1 ) and D' Peak (1620cm -1 ) ( Figure 2).
  • fluorescence spectroscopy defects can cause fluorescence peaks in two-dimensional materials, and the defects can be analyzed according to the positions of the fluorescence peaks.
  • the electron beam used in the TEM characterization process is relatively high and causes new defects.
  • the characterization area is relatively small, the sample preparation requirements are harsh, the cost is high, and the efficiency is low; the use of Raman to characterize the defect samples
  • the laser spot for Raman characterization is on the order of micrometers, and the efficiency is low, and large-area Raman scanning cannot be performed; when using fluorescence spectroscopy to characterize two-dimensional materials, most of them need to be at low temperature (low temperature is sensitive to defects) )get on.
  • One of the objectives of the present application is to provide a method for characterizing defects in two-dimensional materials, which is fast and intuitive, and can be characterized at room temperature, which is a non-destructive method.
  • the second purpose of this application is to provide an application of a method for characterizing two-dimensional material defects in the detection of two-dimensional material-based components.
  • a method for characterizing two-dimensional material defects which includes the following steps:
  • Fluorescence lifetime imaging is performed on the defect-free two-dimensional material substrate sample and the two-dimensional material substrate sample to be tested independently under the same excitation wavelength, and judge whether there is a defect according to the change of the fluorescence lifetime: if the two-dimensional material to be tested The fluorescence lifetime of the material substrate sample is higher than the fluorescence lifetime of the non-defective two-dimensional material substrate sample, the two-dimensional material substrate sample to be tested is a defective sample; if the fluorescence lifetime of the two-dimensional material substrate sample to be tested is different from that of the non-defective two-dimensional material substrate sample The fluorescence lifetime of the defective two-dimensional material substrate sample has no obvious change compared with that, and the two-dimensional material substrate sample to be tested is a non-defective sample.
  • the fluorescence lifetime of step (b) is obtained through a fluorescence lifetime image or a fluorescence lifetime decay curve.
  • step (b) further includes judging the number of defects according to the degree of change in the fluorescence lifetime: the fluorescence lifetime of the two-dimensional material substrate sample to be tested and the defect-free two-dimensional material substrate The greater the difference between the fluorescence lifetimes of the samples, the greater the number of defects in the samples.
  • step (b) performing fluorescence lifetime imaging on the sample by the fluorescence lifetime imaging system includes the following steps:
  • the laser emits laser light, the laser passes through the galvanometer, and then is reflected by the beam splitter to the objective lens to focus on the sample.
  • the optical signal generated by the sample is collected by the objective lens and transmitted through the beam splitter, and then the filter is used to detect the fluorescence lifetime of the sample;
  • the detector detects the light signal, and then uses the time-correlated single photon counting system to synchronize the photodetector and the laser, and obtains the fluorescence lifetime image through the galvanometer scanning.
  • the excitation wavelength of the laser is 450-500nm, and the excitation frequency is 35-45MHz;
  • the wavelength of the filter is 500-700nm
  • the resolution of the time-dependent single-photon counting system is 6-10 ps.
  • the two-dimensional material includes a two-dimensional material directly grown on a substrate by chemical vapor deposition, or transferred to the substrate by a mechanical lift-off or photoresist transfer method.
  • Two-dimensional material on a substrate by chemical vapor deposition or transferred to the substrate by a mechanical lift-off or photoresist transfer method.
  • the two-dimensional material comprising a transition metal sulfides, transition metal or a transition metal selenides, tellurides, preferably comprising WS 2, MoS 2, ReS 2 , WSe 2, MoSe 2, Bi 2 Se 3, MoTe 2, WTe 2 or one of Bi 2 Te 3 .
  • the substrate includes a metallic copper, nickel, platinum, iron or alloy substrate.
  • the defects include one or more of point defects, grain boundary defects, wrinkles or broken edges.
  • the two-dimensional material-based element includes a diode, a spin element, a field effect transistor or a tunneling transistor.
  • This application uses the fluorescence lifetime imaging method to characterize the defects of two-dimensional materials. This method can quickly and intuitively observe the changes in the fluorescence lifetime to determine whether the material is defective, and the fluorescence lifetime imaging method is not sensitive to temperature and can be characterized at room temperature , And will not introduce new defects, it is a non-destructive testing method with large characterization area, fast imaging and high efficiency.
  • FIG. 1 is a schematic diagram of a method for characterizing two-dimensional material defects using TEM in the prior art
  • Figure 2 is a schematic diagram of a prior art method for characterizing two-dimensional material defects using spectroscopy (where (a) is a schematic diagram of a method for characterizing graphene defects using Raman spectroscopy, and (b) is a schematic diagram of using fluorescence spectroscopy to characterize the fluorescence peaks generated by WSe 2 defects) ;
  • FIG. 3 is a schematic structural diagram of a fluorescence lifetime imaging system according to an embodiment of this application.
  • Figure 4 shows the fluorescence lifetime images of the WS 2 sample before and after plasma treatment in Example 1 at an excitation wavelength of 561 nm (where (a) is the fluorescence lifetime image of the WS 2 sample before plasma treatment at an excitation wavelength of 561 nm, and (b) is after plasma treatment Fluorescence lifetime image of WS 2 sample at 561nm excitation wavelength);
  • Figure 5 is the fluorescence spectrum of the original monolayer WS 2 before and after plasma treatment
  • Figure 6 is the time-resolved fluorescence lifetime decay curve of the original and defective WS 2 samples (where (a) is the time-resolved fluorescence lifetime decay curve of the original WS 2 sample, and (b) is the time-resolved WS 2 sample with defects Fluorescence lifetime decay curve);
  • Figure 7 shows the influence of different excitons on exciton-exciton annihilation (where (a) is the relationship between the fluorescence spectrum weight of neutral exciton, trion exciton and defect state exciton and laser power, and (b) is the original and defects with a single layer of neutral excitons WS 2, trion exciton lifetime bi-exponential function fitting time-resolved fluorescence decay lifetime graph, (c) a single layer and with the original WS defective 2 The time-resolved fluorescence lifetime decay curve fitted by the double exponential function of the life weight of sex exciton and trion exciton, (d) is the average lifetime of the original and defective WS 2 under different excitation intensities, and (e) is the original The relationship between steady-state fluorescence intensity of neutral excitons and laser power in single-layer WS 2 with defects);
  • Figure 8 shows the fluorescence lifetime imaging images of neutral and defect state excitons under different excitation intensities (where (a) is the fluorescence lifetime imaging images of the neutral excitons in the original monolayer WS 2 under different excitation intensities, and (b) is different The fluorescence lifetime imaging images of the neutral excitons in the defective single-layer WS 2 under excitation intensity, (c) is the fluorescence lifetime imaging images of the defective excitons in the defective single-layer WS 2 under different excitation intensities);
  • Figure 9 is a time-resolved fluorescence lifetime decay curve diagram of the original single-layer WS 2 neutral excitons under different excitation intensities
  • Figure 10 is a time-resolved fluorescence lifetime decay curve diagram of neutral excitons in a single-layer WS 2 with defects under different excitation intensities;
  • Fig. 11 is a graph showing the time-resolved fluorescence lifetime decay curve of defective excitons in a single-layer WS 2 with defects under different excitation intensities;
  • Figure 12 is a linear graph after fitting the time-resolved fluorescence lifetime decay curve of the neutral excitons in the original monolayer WS 2 under different excitation intensities;
  • Fig. 13 is a linear graph after fitting the time-resolved fluorescence lifetime decay curve of neutral excitons in a single-layer WS 2 with defects under different excitation intensities;
  • Fig. 14 is a linear graph after fitting the time-resolved fluorescence lifetime decay curve of defect state excitons in a single-layer WS 2 with defects under different excitation intensities;
  • FIG 15 is a single original WS 2 and WS 2 monolayers with defects excitons EEA process kinetics and FIG.
  • a method for characterizing defects in a two-dimensional material which includes the following steps:
  • Fluorescence lifetime imaging is performed on the defect-free two-dimensional material substrate sample and the two-dimensional material substrate sample to be tested independently under the same excitation wavelength, and judge whether there is a defect according to the change of the fluorescence lifetime: if the two-dimensional material to be tested The fluorescence lifetime of the material substrate sample is higher than the fluorescence lifetime of the non-defective two-dimensional material substrate sample, the two-dimensional material substrate sample to be tested is a defective sample; if the fluorescence lifetime of the two-dimensional material substrate sample to be tested is different from that of the non-defective two-dimensional material substrate sample The fluorescence lifetime of the defective two-dimensional material substrate sample has no obvious change compared with that, and the two-dimensional material substrate sample to be tested is a non-defective sample.
  • Two-dimensional materials refer to materials in which electrons can only move freely (planar motion) in two dimensions of non-nanoscale (1-100nm).
  • the two-dimensional material here refers to a two-dimensional material that can emit fluorescence.
  • transition metal sulfides transition metal selenide or a transition metal telluride, including but not limited to WS 2, MoS 2, ReS 2 , WSe 2, MoSe 2, Bi 2 Se 3, MoTe 2
  • the two-dimensional material is tungsten disulfide (WS 2 ).
  • the source of the two-dimensional material is not limited, and it may be a two-dimensional material directly grown on the substrate by chemical vapor deposition, or a two-dimensional material transferred to the substrate by mechanical lift-off or photoresist transfer.
  • Mechanical peeling is the use of mechanical force to peel a single layer of 2D material from the surface of the 2D material crystal, that is, a layer of 2D material can be peeled off from the 2D material directly with tape, and then repeatedly pasted between the tapes to make the 2D material The layer becomes thinner and thinner, and then the tape is attached to the substrate, and the single layer of two-dimensional material is transferred to the substrate.
  • defects are not limited, including but not limited to grain boundary defects, wrinkles, broken edges, or point defects.
  • the substrate is not limited, and can be, but is not limited to, a metal copper, nickel, platinum, iron or alloy substrate, and can be a base material or a flexible base used in a semiconductor manufacturing process, and can be determined according to the final element to be prepared.
  • 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.
  • a non-defective two-dimensional material substrate sample is used as a reference product, and the fluorescence lifetime of the reference product at a certain excitation wavelength is tested by fluorescence lifetime imaging as a standard.
  • the two-dimensional material substrate sample to be tested is subjected to fluorescence lifetime imaging under the same conditions (same excitation wavelength), and the fluorescence lifetime of the reference product is compared to determine whether there is a defect.
  • the two-dimensional material substrate sample to be tested is a defective sample; if the fluorescence lifetime of the two-dimensional material substrate sample to be tested is the same as that of the reference product There is no significant change in the fluorescence lifetime, and the two-dimensional material substrate sample to be tested is a defect-free sample.
  • Fluorescence lifetime refers to the average time that fluorescence stays in the excited state, about the order of ns. Fluorescence lifetime is a characteristic of the molecule itself and has nothing to do with the concentration of the fluorophore and the excitation light intensity.
  • Fluorescence lifetime imaging fluorescence lifetime microscopic imaging
  • Fluorescence lifetime measurement methods include frequency domain method and time domain method. Time domain method is preferred. Time domain method is also called pulse method. Fluorescence lifetime is calculated by using ultrashort pulse laser to excite fluorescent sample, and then measuring the intensity decay curve of sample fluorescence. A non-limiting method is Time-Correlated Single Photon Counting (TCSPC).
  • TCSPC Time-Correlated Single Photon Counting
  • the working principle of lifetime measurement based on the TCSPC method is: excite the sample with high-repetition pulse excitation light, and in each pulse period, Excite fluorescent molecules to emit one photon at most, so at most one photon can be detected in each cycle, and then record the moment when the photon appears, and record one photon at that moment, and the same situation is the same in the next pulse cycle, after many times Counting can obtain the distribution curve of fluorescence photons over time, which is equivalent to the fluorescence decay curve.
  • the fluorescence lifetime of the sample can be obtained by fitting the decay curve or analyzing other forms of data.
  • fluorescence lifetime image can directly indicate the fluorescence lifetime
  • fluorescence lifetime decay curve can be obtained by fitting or other forms of data analysis to obtain the fluorescence lifetime of the sample.
  • the different colors on the fluorescence lifetime image represent the length of the fluorescence lifetime, and the change in the fluorescence lifetime of the material can be seen directly. That is, the fluorescence lifetime can be visually judged according to the respective fluorescence lifetime imaging images of the reference substance and the two-dimensional material substrate sample to be tested, and the sample with a relatively long fluorescence lifetime is a two-dimensional material substrate sample with defects.
  • the fluorescence lifetime decay curve of this region is obtained, and then the fluorescence lifetime of this region can be obtained by fitting or other forms of data analysis.
  • the fluorescence lifetime of the entire sample is calculated as the average value of the fluorescence lifetime of each region. Therefore, if the average fluorescence lifetime of the two-dimensional material substrate sample to be tested is higher than the average fluorescence lifetime of the reference product, the two-dimensional material substrate sample to be tested is a defective sample; if the average fluorescence lifetime of the two-dimensional material substrate sample to be tested is Compared with the average fluorescence lifetime of the reference product, there is no significant change, and the two-dimensional material substrate sample to be tested is a defect-free sample.
  • Light excitation makes the electron transition from the ground state to the excited state, and the electron relaxes and recombines with the hole to emit photons.
  • a defect fluorescence peak can be generated.
  • the defect fluorescence lifetime is longer than the inherent fluorescence lifetime, so the average fluorescence lifetime of the defective sample becomes longer.
  • This application uses a fluorescence lifetime imaging method to characterize two-dimensional material defects. This method judges whether the material has defects according to the change in fluorescence lifetime, and characterizes the influence of the defect on the fluorescence lifetime through the imaging method, so that defects can be detected more quickly and intuitively.
  • the fluorescence lifetime imaging method is not sensitive to temperature and can be characterized at room temperature without introducing new defects. It is a non-destructive testing method with a large characterization area, fast imaging, and high efficiency.
  • step (b) further includes judging the number of defects according to the degree of change in fluorescence lifetime.
  • the degree of change in fluorescence lifetime refers to the difference in fluorescence lifetime between the sample to be tested and the reference, that is, the two-dimensional material liner to be tested. The greater the difference between the fluorescence lifetime of the bottom sample and the fluorescence lifetime of the defect-free two-dimensional material substrate sample, the greater the number of defects in the sample.
  • 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 degree, and the more defects introduced by the two-dimensional material substrate sample. Based on this, the number of defects can be roughly judged.
  • the composition of a TCSPC FLIM system generally includes: a confocal microscopy imaging system, TCSPC counter, FLIM detector, and analysis software.
  • a typical FLIM system is shown in Figure 3, including a laser (laser driver 1, laser head 2), galvanometer 3, beam splitter 4, objective lens 5, sample 6, filter 7, and photodetector 8.
  • laser driver 1 laser driver 1
  • galvanometer 3 galvanometer 3
  • beam splitter 4 objective lens 5
  • sample 6, filter 7 sample 6, filter 7, and photodetector 8.
  • TCSPC 9 the process of fluorescence lifetime imaging through this system is as follows:
  • the laser emits laser light, and the laser passes through the galvanometer 3, and is reflected by the beam splitter 4 to the objective lens 5 to focus on the sample 6.
  • the optical signal generated by the sample is collected by the objective lens 5 and then transmitted through the beam splitter 4, and then the fluorescence of the sample is treated by the filter 7
  • the lifetime is detected; then the photodetector 8 is used to detect the optical signal, and then the TCSPC 9 is used to synchronize the photodetector and the laser, and the fluorescence lifetime image is obtained by scanning through the galvanometer.
  • the excitation wavelength of the laser is 450-500nm, such as 450nm, 460nm, 470nm, 488nm, 495nm or 500nm
  • the excitation frequency is 35-45MHz, such as 35MHz, 36MHz, 37MHz, 38MHz, 39MHz, 40MHz, 41MHz, 42MHz, 43MHz , 44MHz or 45MHz.
  • the wavelength of the filter is 500-700nm, such as 500nm, 550nm, 561nm, 600nm, 624nm, 650nm or 700nm.
  • the resolution of the time-dependent single photon counting system is 6-10 ps, such as 6 ps, 7 ps, 8 ps, 9 ps or 10 ps.
  • the two-dimensional material-based element includes a diode, a spin element, a field-effect transistor, or a tunneling transistor.
  • a diode is an electronic component with two electrodes that only allows current to flow in a single direction;
  • a spin component is an electronic component with spin properties;
  • a field effect transistor uses the electric field effect of the control input circuit to A semiconductor component that controls the output loop current;
  • a tunneling transistor is a crystal component with a tunneling effect.
  • the method for characterizing defects in two-dimensional materials of the present application can be used to detect whether a sample based on two-dimensional materials has defects. Since the defects of two-dimensional materials can affect the performance of components, it is of great significance to detect whether two-dimensional materials have defects.
  • the method can quickly select defective two-dimensional material-based components, avoiding time-consuming electrical measurement methods to determine component performance.
  • test methods used in the following examples are conventional methods unless otherwise specified, and the raw materials and reagents used, unless otherwise specified, are all raw materials and reagents available from conventional commercial channels such as commercially available.
  • the two-dimensional material WS 2 was prepared on the peeling substrate by the mechanical peeling method.
  • Prepare samples with defects use a plasma cleaner to introduce defects into the sample.
  • the power of the plasma cleaner is 20W
  • the radio frequency is 13.56MH
  • the sample is bombarded with argon for 10s.
  • the excitation wavelength is 488nm
  • the frequency is 40MHz
  • the objective lens 40X, NA 0.95
  • the generated fluorescence signal is collected by the same objective lens, and the fluorescence lifetime of the original and defective monolayer WS 2 is filtered by a 561nm long pass filter.
  • the filtered optical signal is detected by a photodetector, and then time correlation is used
  • the single photon counting system (TSSPC) synchronizes the photodetector and the laser to obtain the fluorescence lifetime of each spot position, and finally obtains the fluorescence lifetime image through the galvanometer scan.
  • TCSPC resolution is 8.0ps.
  • the fluorescence spectrum of the original single layer WS 2 is shown in Figure 5. It can be seen that the fluorescence peak of WS 2 before plasma treatment is composed of the neutral exciton peak and the trion peak. After plasma treatment, a new peak appears, that is, the defect state. Exciton peak.
  • Example 3 The effect of defects on the exciton-exciton annihilation process in a single-layer tungsten disulfide
  • Exciton-exciton annihilation is a process in which an exciton transfers energy to another exciton. It is a non-radiative process, so it can shorten the fluorescence lifetime, and the exciton annihilation rate can be obtained through data processing.
  • the intrinsic excitons in the sample can be bound to form defective excitons, which will reduce the number of intrinsic excitons, so the number of excitons used to participate in the exciton annihilation process is reduced, and the annihilation rate is reduced.
  • Figure 7 shows the influence of different excitons on exciton-exciton annihilation.
  • Figure 7(a) shows the fluorescence spectrum weights of neutral excitons, trion excitons and defect state excitons as a function of laser power. All fluorescence The spectrum values are all normalized to the peak value. The study found that the contribution rate of the neutral exciton peak decreases, and the other is the opposite of the decrease trend of the contribution rate of the neutral exciton peak. This is because the decrease of the neutral exciton peak leads to an increase in the trion exciton and defect state exciton peaks. At the same time, trion excitons have no significant contribution to defect state excitons, indicating that only neutral excitons are bound by defects.
  • the average lifetime of the original sample and the defective sample in Figure 7 (d) decreases with the increase of excitation intensity, indicating that the nonlinear attenuation channel EEA becomes the dominant relaxation channel under high excitation intensity and recombines with radiation Channel competition leads to shortening of exciton life.
  • the fluorescence lifetime of the defective sample is longer than that of the original sample, indicating that the defect caused the reduction of EEA to some extent.
  • the dependence of the peak intensity of the neutral exciton on the laser power is plotted. At low laser power, the integrated fluorescence intensity of the original monolayer and the defective monolayer are closed, indicating that the defect is neutral The effect of exciton dynamics is small.
  • Figure 8 is the fluorescence lifetime imaging diagram of the neutral and defect state excitons under different excitation intensities. As shown in Fig. 8 (a), (b), (c), we find that the exciton lifetime ratio under high excitation intensity is low The exciton life under excitation intensity is short. The same result is also observed for the defect state excitons in Fig. 8(c), which shows that EEA occurs in single layer WS 2 under high excitation intensity. In addition, we normalized the time-resolved fluorescence lifetime decay curves ( Figures 9-11) under different excitation intensities to obtain Figures 12-14, indicating that the fluorescence lifetime dynamics strongly depend on different initial excitation densities n( 0) The corresponding excitation intensity. Figure 12-14 uses formula (1) to perform TRPL curve linearization data on neutral excitons and defective state excitons, and the solid line is a linear fit.
  • n(t) is the exciton group
  • n 0 is the initial exciton density
  • t is the decay time
  • k 0 1/ ⁇ 0 is the intrinsic exciton recombination rate
  • ⁇ 0 is the PL lifetime or defect confinement of neutral excitons at low excitation intensity without exciton-exciton annihilation
  • ⁇ is the exciton annihilation rate constant, which is assumed to be independent of the decay time.
  • the exciton density is estimated using an absorption coefficient of 3.5%, and the energy of each pulse is calculated.
  • the initial exciton density is related to neutral excitons, and each photon can excite one exciton.
  • the decay trend develops rapidly, where the decay signal of the initial exciton density below has relatively slow relaxation dynamics.
  • the EEA in the single layer WS 2 can be triggered.
  • exciton-exciton annihilation (EEA) controls the exciton dynamics leading to faster decay, which is an additional important non-radiative relaxation channel.
  • the defect After introducing the defect, some excited neutral excitons are captured by the defect. Because the introduced defect can restrain the excitons, the number of excitons used to generate EEA is reduced, and therefore the EEA rate is reduced. This situation is similar to one-dimensional organic Self-trapping of excitons in metal halide nanotubes.
  • the EEA rate is related to exciton diffusion, and defects can inhibit exciton diffusion.
  • the defect state excitons also promote radiation relaxation and become an energy dissipation path that competes with the exciton annihilation process.
  • the EEA rate of defect 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 the binding energy of the neutral excitons in the single layer, which leads to more non-locality and faster exciton diffusion.
  • the exciton diffusion constant is proportional to the diffusion length. The longer the defect state exciton diffusion length, the longer the time for EEA to occur when two excitons are close to each other.

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Abstract

一种表征二维材料缺陷的方法及其应用，涉及纳米材料缺陷表征技术领域。表征缺陷的方法包括：分别独立地对无缺陷的二维材料衬底样品和待测二维材料衬底样品进行荧光寿命成像，根据荧光寿命的变化判断有无缺陷：如果待测二维材料衬底样品的荧光寿命高于无缺陷的二维材料衬底样品的荧光寿命，则待测二维材料衬底样品为有缺陷样品；如果待测二维材料衬底样品的荧光寿命与无缺陷的二维材料衬底样品的荧光寿命相比无明显变化，则待测二维材料衬底样品为无缺陷样品。该方法采用荧光寿命成像方法表征二维材料缺陷，能够快速、直观地观察荧光寿命变化，从而判断材料有无缺陷，在室温下即可表征，不会引入新的缺陷，是一种无损检测方法。

Description

表征二维材料缺陷的方法及其应用
交叉引用
本申请引用于2019年01月31日提交的专利名称为“表征二维材料缺陷的方法及其应用”的第2019101014786号中国专利申请，其通过引用被全部并入本申请。
技术领域
本申请涉及纳米材料缺陷表征技术领域，具体而言，涉及一种表征二维材料缺陷的方法及其应用。
背景技术
二维材料由于具有许多独特的电学、光学、化学以及热学特性，使得二维材料有着广泛应用，例如在微电子及光电子元器件、半导体器件以及太阳能电池的构筑中起着非常重要的作用。同时，二维材料适合作为研究材料结构和物理性质的载体，也可以作为构筑其他维度材料的基础结构单元，因为对于二维材料的研究十分重要。
然而，二维材料难以在自然中大量存在，一般都是通过人工手段从自然材料中剥离或是通过其他物质合成，而采用各种方式制得的二维材料都不可避免的存在一定的缺陷，这些缺陷会严重影响元件的性能。因此，缺陷的表征识别手段显得尤为重要。
目前常见的表征手段主要有两种：透射电子显微镜(TEM)和光谱学表征。TEM的分辨率一般都在纳米级别，可以观察到原子结构，一般通过对表面晶格图像进行滤波处理后可以观察到原子像，所以通过高分辨TEM图像可以看到缺陷(如图1)。光谱学表征主要包括拉曼和荧光光谱，例如利用拉曼光谱识别石墨烯时，石墨烯中的缺陷使拉曼光谱中出现两个新的振动模式，即D峰(1350cm -1)和D’峰(1620cm -1)(如图2)。对于荧光光谱法，缺陷可以使二维材料中产生荧光峰，根据荧光峰出现的位置可以分析缺陷。
但这些方法都存在一些不足：TEM表征过程中使用的电子束较高而造 成新的缺陷，此外表征面积也比较小，样品制备要求苛刻，成本高、效率低；利用拉曼对缺陷样品的表征时间长，拉曼表征的雷射光斑为微米量级，效率较低，不能进行大面积的拉曼扫描表征；利用荧光光谱表征二维材料时大多数需要在低温下(低温下才对缺陷敏感)进行。
因此，所期望的是提供一种新型的表征二维材料缺陷的方法，其能够解决上述问题中的至少一个。
有鉴于此，特提出本申请。
发明内容
本申请的目的之一在于提供一种表征二维材料缺陷的方法，通过荧光寿命成像方法表征缺陷，快速、直观，可在室温下表征，是一种无损方法。
本申请的目的之二在于提供一种表征二维材料缺陷的方法在检测基于二维材料的元件中的应用。
为了实现本申请的上述目的，特采用以下技术方案：
第一方面，提供了一种表征二维材料缺陷的方法，包括以下步骤：
(a)提供无缺陷的二维材料衬底样品和待测二维材料衬底样品；
(b)在同一激发波长下分别独立地对无缺陷的二维材料衬底样品和待测二维材料衬底样品进行荧光寿命成像，根据荧光寿命的变化判断有无缺陷：如果待测二维材料衬底样品的荧光寿命高于无缺陷的二维材料衬底样品的荧光寿命，则待测二维材料衬底样品为有缺陷样品；如果待测二维材料衬底样品的荧光寿命与无缺陷的二维材料衬底样品的荧光寿命相比无明显变化，则待测二维材料衬底样品为无缺陷样品。
优选地，在本申请提供的技术方案的基础上，步骤(b)的荧光寿命通过荧光寿命图像或荧光寿命衰减曲线获得。
优选地，在本申请提供的技术方案的基础上，步骤(b)还包括根据荧光寿命的变化程度判断缺陷数量：待测二维材料衬底样品的荧光寿命与无缺陷的二维材料衬底样品的荧光寿命之间的差值越大，样品的缺陷数量越多。
优选地，在本申请提供的技术方案的基础上，步骤(b)中，通过荧光寿命成像系统对样品进行荧光寿命成像，包括以下步骤：
激光器发射激光，激光通过振镜，再由分束镜反射至物镜聚焦至样品， 样品产生的光信号由物镜采集后透过分束镜，再用滤波片对样品的荧光寿命进行检测；然后用光电探测器检测光信号，再利用时间相关单光子计数系统对光电探测器和激光器进行同步，通过振镜扫描得到荧光寿命图像。
优选地，在本申请提供的技术方案的基础上，激光器的激发波长为450-500nm，激发频率为35-45MHz；
优选地，滤波片的波长为500-700nm；
优选地，时间相关单光子计数系统的分辨率为6-10ps。
优选地，在本申请提供的技术方案的基础上，所述二维材料包括经化学气相沉积直接生长在衬底上的二维材料，或，经机械剥离或光刻胶转移方法转移到衬底上的二维材料；
优选地，所述二维材料包括过渡金属硫化物、过渡金属硒化物或过渡金属碲化物，优选包括WS 2、MoS 2、ReS 2、WSe 2、MoSe 2、Bi 2Se 3、MoTe 2、WTe 2或Bi 2Te 3中的一种。
优选地，在本申请提供的技术方案的基础上，所述衬底包括金属铜、镍、铂、铁或合金衬底。
优选地，在本申请提供的技术方案的基础上，所述缺陷包括点缺陷、晶界线缺陷、褶皱或破损边缘中的一种或几种。
第二方面，提供了一种上述表征二维材料缺陷的方法在检测基于二维材料的元件中的应用。
优选地，所述基于二维材料的元件包括二极体、自旋元件、场效电晶体或穿隧电晶体。
与已有技术相比，本申请具有如下有益效果：
本申请通过采用荧光寿命成像方法表征二维材料缺陷，该方法能够快速、直观地观察荧光寿命变化，从而判断材料有无缺陷，而且荧光寿命成像方法对温度不敏感，在室温下即可进行表征，而且不会引入新的缺陷，是一种无损检测方法，表征面积大，成像快速、效率高。
附图说明
为了更清楚地说明本申请实施例的技术方案，下面将对实施例中所需要使用的附图作简单地介绍，应当理解，以下附图仅示出了本申请的某些实施例，因此不应被看作是对范围的限定，对于本领域普通技术人员来讲， 在不付出创造性劳动的前提下，还可以根据这些附图获得其他相关的附图。
图1为现有技术利用TEM表征二维材料缺陷方法示意图；
图2为现有技术利用光谱法表征二维材料缺陷方法示意图(其中(a)为利用拉曼光谱表征石墨烯缺陷方法示意图，(b)为利用荧光光谱表征WSe 2缺陷产生的荧光峰示意图)；
图3为本申请一种实施方式的荧光寿命成像系统的结构示意图；
图4为实施例1等离子处理前后的WS 2样品561nm激发波长下的荧光寿命图像(其中(a)为等离子处理前的WS 2样品561nm激发波长下的荧光寿命图像，(b)为等离子处理后的WS 2样品561nm激发波长下的荧光寿命图像)；
图5为等离子处理前后的原始单层WS 2的荧光谱图；
图6为原始的和带缺陷的WS 2样品时间分辨荧光寿命衰减曲线图(其中(a)为原始的WS 2样品时间分辨荧光寿命衰减曲线图，(b)为带缺陷的WS 2样品时间分辨荧光寿命衰减曲线图)；
图7为不同激子对激子-激子湮灭的影响(其中(a)为中性激子、trion激子和缺陷态激子的荧光谱权重与激光功率的关系图，(b)为原始的和带缺陷的单层WS 2中中性激子、trion激子寿命的双指数函数拟合的时间分辨荧光寿命衰减曲线图，(c)为原始的和带缺陷的单层WS 2中中性激子、trion激子寿命重量的双指数函数拟合的时间分辨荧光寿命衰减曲线图，(d)为不同激发强度下原始的和带缺陷的WS 2的平均寿命图，(e)为原始的和带缺陷的单层WS 2中中性激子稳态荧光强度与激光功率的关系图)；
图8为不同激发强度下中性和缺陷态激子的荧光寿命成像图(其中(a)为不同激发强度下原始单层WS 2中中性激子的荧光寿命成像图，(b)为不同激发强度下带缺陷的单层WS 2中中性激子的荧光寿命成像图，(c)为不同激发强度下带缺陷的单层WS 2中缺陷态激子的荧光寿命成像图)；
图9为不同激发强度下原始的单层WS 2中中性激子的时间分辨荧光寿命衰减曲线图；
图10为不同激发强度下带缺陷的单层WS 2中中性激子的时间分辨荧光寿命衰减曲线图；
图11为不同激发强度下带缺陷的单层WS 2中缺陷态激子的时间分辨 荧光寿命衰减曲线图；
图12为不同激发强度下原始单层WS 2中中性激子的时间分辨荧光寿命衰减曲线拟合后的线性图；
图13为不同激发强度下带缺陷的单层WS 2中中性激子的时间分辨荧光寿命衰减曲线拟合后的线性图；
图14为不同激发强度下带缺陷的单层WS 2中缺陷态激子的时间分辨荧光寿命衰减曲线拟合后的线性图；
图15为原始单层WS 2和带缺陷的单层WS 2中激子动力学和EEA过程图。
图示：1-激光驱动器；2-激光头；3-振镜；4-分束镜；5-物镜；6-样品；7-滤波片；8-光电探测器；9-TCSPC。
具体实施方式
下面将结合实施例对本申请的实施方案进行详细描述，但是本领域技术人员将会理解，下列实施例仅用于说明本申请，而不应视为限制本申请的范围。实施例中未注明具体条件者，按照常规条件或制造商建议的条件进行。所用试剂或仪器未注明生产厂商者，均为可以通过市售购买获得的常规产品。
根据本申请的第一个方面，提供了一种表征二维材料缺陷的方法，包括以下步骤：
(a)提供无缺陷的二维材料衬底样品和待测二维材料衬底样品；
(b)在同一激发波长下分别独立地对无缺陷的二维材料衬底样品和待测二维材料衬底样品进行荧光寿命成像，根据荧光寿命的变化判断有无缺陷：如果待测二维材料衬底样品的荧光寿命高于无缺陷的二维材料衬底样品的荧光寿命，则待测二维材料衬底样品为有缺陷样品；如果待测二维材料衬底样品的荧光寿命与无缺陷的二维材料衬底样品的荧光寿命相比无明显变化，则待测二维材料衬底样品为无缺陷样品。
二维材料是指电子仅可在两个维度的非纳米尺度(1-100nm)上自由运动(平面运动)的材料。
可以理解的是，这里的二维材料是指能发射荧光的二维材料。典型但非限制性的例如为过渡金属硫化物、过渡金属硒化物或过渡金属碲化物等， 包括但不限于WS 2、MoS 2、ReS 2、WSe 2、MoSe 2、Bi 2Se 3、MoTe 2、WTe 2或Bi 2Te 3中的一种，作为一种典型的示例性方案，二维材料为二硫化钨(WS 2)。
对二维材料的来源不作限定，可以是经化学气相沉积直接生长在衬底上的二维材料，也可以是经机械剥离或光刻胶转移方法转移到衬底上的二维材料。
机械剥离是利用机械力从二维材料晶体的表面剥离出单层二维材料，即可以直接用胶带从二维材料上揭下一层二维材料，然后在胶带之间反复粘贴使二维材料片层越来越薄，再将胶带贴在衬底上，单层二维材料即转移到衬底上。
对缺陷的种类不作限定，包括但不限于晶界线缺陷、褶皱、破损边缘或点缺陷等。
对衬底不作限定，可以是但不限于金属铜、镍、铂、铁或合金衬底，可以是半导体制程所用的基底材料或柔性基底，可根据最终想要制备的元件而定。
无缺陷的二维材料衬底样品是指具有完美二维材料的衬底，即衬底样品上二维材料是没有缺陷的完美二维材料。
本申请将无缺陷的二维材料衬底样品作为对照品，通过荧光寿命成像测试对照品某一激发波长下的荧光寿命，以此作为标准。
当检测待测样品时，将待测二维材料衬底样品在相同条件(同一激发波长)下进行荧光寿命成像，通过与对照品的荧光寿命进行比较，来判断有无缺陷。
如果待测二维材料衬底样品的荧光寿命高于对照品的荧光寿命，则待测二维材料衬底样品为有缺陷样品；如果待测二维材料衬底样品的荧光寿命与对照品的荧光寿命相比无明显变化，则待测二维材料衬底样品为无缺陷样品。
荧光寿命是指荧光在激发态停留的平均时间，约为ns量级，荧光寿命是分子本身的特性，与荧光团的浓度和激发光强无关。
荧光寿命成像(荧光寿命显微成像)技术是将显微镜与寿命测量技术相结合用于研究荧光的寿命。荧光寿命测量方法包括频域法和时域法，优 选时域法，时域法又称脉冲法，通过使用超短脉冲激光激发荧光样品，然后测量样品荧光的强度衰减曲线计算荧光寿命，典型但非限制性的方法为时间相关单光子计数法(Time-Correlated Single Photon Counting，TCSPC)，基于TCSPC方法的寿命测量的工作原理为：使用高重复脉冲激发光激发样品，在每一个脉冲周期内，最多激发荧光分子发出一个光子，因此在每个周期内最多能探测到一个光子，然后记录光子出现的时刻，并在该时刻记录一个光子，再下一个脉冲周期内也是相同的情况，经过多次计数可以得到荧光光子随时间的分布曲线，即等效于荧光衰减曲线，通过对衰减曲线进行拟合或其他形式数据分析可以得到样品的荧光寿命。
通过荧光寿命成像测试能够获得两类数据：荧光寿命图像或荧光寿命衰减曲线。荧光寿命图像可以直观表示荧光寿命，荧光寿命衰减曲线通过拟合或其他形式数据分析得到样品的荧光寿命。
荧光寿命图像上不同颜色代表荧光寿命的长短，可以直观看到材料荧光寿命的改变。即根据对照品和待测二维材料衬底样品各自的荧光寿命成像图直观判断荧光寿命，荧光寿命相对长的样品为带有缺陷的二维材料衬底样品。
通过对荧光寿命图像上某一区域进行放大，获得该区域的荧光寿命衰减曲线，再通过拟合或其他形式数据分析得到该区域的荧光寿命。
需要注意的是，整个样品的荧光寿命以各个区域荧光寿命的平均值计算。因此如果待测二维材料衬底样品的平均荧光寿命高于对照品的平均荧光寿命，则待测二维材料衬底样品为有缺陷样品；如果待测二维材料衬底样品的平均荧光寿命与对照品的平均荧光寿命相比无明显变化，则待测二维材料衬底样品为无缺陷样品。
无明显变化是指荧光寿命浮动范围在±5％。
光激发使电子从基态跃迁到激发态，电子弛豫与空穴复合，发射光子。当缺陷存在时，可以产生缺陷荧光峰，通常缺陷荧光寿命比固有荧光寿命长，因此造成缺陷样品的平均荧光寿命变长。
本申请采用荧光寿命成像方法表征二维材料缺陷，该方法根据荧光寿命变化判断材料有无缺陷，通过成像方法表征缺陷对荧光寿命的影响，可以更快速直观检测缺陷。此外，荧光寿命成像方法对温度不敏感，在室温 下即可进行表征，而且不会引入新的缺陷，是一种无损检测方法，表征面积大，成像快速、效率高。
在一种实施方式中，步骤(b)还包括根据荧光寿命的变化程度判断缺陷数量，荧光寿命的变化程度是指待测样品与对照品荧光寿命的差值大小，即待测二维材料衬底样品的荧光寿命与无缺陷的二维材料衬底样品的荧光寿命之间的差值越大，样品的缺陷数量越多。
荧光寿命的长短与样品的缺陷数量呈负相关，衰减速率的快慢与样品的缺陷数量呈正相关，即荧光寿命越短，衰减程度越快，二维材料衬底样品引入的缺陷越多。据此，可以粗略判断缺陷的数量。
在一种实施方式中，一个TCSPC FLIM系统的组成一般包含：一个共聚焦显微成像系统、TCSPC计数器、FLIM探测器和分析软件。
优选地，一种典型的FLIM系统如图3所示，包括激光器(激光驱动器1、激光头2)、振镜3、分束镜4、物镜5、样品6、滤波片7、光电探测器8和TCSPC 9，通过该系统进行荧光寿命成像的过程如下：
激光器发射激光，激光通过振镜3，再由分束镜4反射至物镜5聚焦至样品6，样品产生的光信号由物镜5采集后透过分束镜4，再用滤波片7对样品的荧光寿命进行检测；然后用光电探测器8检测光信号，再利用TCSPC 9对光电探测器和激光器进行同步，通过振镜扫描得到荧光寿命图像。
对激光器的波长频率、滤波片的波长和单光子计数系统的分辨率等不作限制。
优选地，激光器的激发波长为450-500nm，例如450nm、460nm、470nm、488nm、495nm或500nm，激发频率为35-45MHz，例如35MHz、36MHz、37MHz、38MHz、39MHz、40MHz、41MHz、42MHz、43MHz、44MHz或45MHz。
优选地，滤波片的波长为500-700nm，例如500nm、550nm、561nm、600nm、624nm、650nm或700nm。
优选地，时间相关单光子计数系统的分辨率为6-10ps，例如6ps、7ps、8ps、9ps或10ps。
在该系统下，优化参数能够使成像效果更好。
根据本申请的第二个方面，提供了一种上述表征二维材料缺陷的方法在检测基于二维材料的元件中的应用。
优选地，基于二维材料的元件包括二极体、自旋元件、场效电晶体或穿隧电晶体。
二极体是一种具有两个电极的装置，只允许电流由单一方向流过的电子元件；自旋元件是具有自旋属性的电子元件；场效电晶体是利用控制输入回路的电场效应来控制输出回路电流的一种半导体元器件；穿隧电晶体是具有隧穿效应的晶体元器件。
本申请的表征二维材料缺陷的方法可以用于检测基于二维材料样品是否具有缺陷，由于二维材料的缺陷会影响元件的性能，因此通过检测二维材料是否具有缺陷具有重要意义，本申请方法可快速挑选出具有缺陷的基于二维材料的元件，避免了通过耗时的电学测量方式来判定元件的性能。
为了进一步了解本申请，下面结合具体实施例对本申请方法和效果做进一步详细的说明。这些实施例仅是对本申请的典型描述，但本申请不限于此。下述实施例中所用的试验方法如无特殊说明，均为常规方法，所使用的原料，试剂等，如无特殊说明，均为可从常规市购等商业途径得到的原料和试剂。
实施例1 荧光寿命成像法检测WS 2缺陷
利用机械剥离法将二维材料WS 2制备在剥离基底上。
制备带有缺陷的样品：利用等离子清洗机在样品中引入缺陷，等离子清洗机功率为20W，射频为13.56MH，采用氩气轰击样品10s。
利用荧光寿命成像系统(PicoHarp 300,PicoQuant)对带有缺陷的样品进行荧光寿命成像检测，如图3所示，激发波长为488nm，频率为40MHz，使用物镜(40X,NA 0.95)来聚焦激光，用于激发样品，产生的荧光信号由同一个物镜采集，利用561nm长通滤波片对原始和缺陷单层WS 2的荧光寿命进行滤波，滤波后的光信号用光电探测器检测，然后利用时间相关单光子计数系统(TSSPC)对光电探测器和激光器进行同步获得每一个光斑位置的荧光寿命，最后通过振镜扫描得到荧光寿命图像，TCSPC分辨率为8.0ps。
原始(等离子处理前)的WS 2样品和带缺陷(等离子处理后)的WS 2 样品561nm激发波长下的荧光寿命图像如图4所示。
由图4可以很直观地看出，不同的荧光寿命用不同颜色表示，带缺陷样品的荧光寿命明显变长。
实施例2 证实缺陷使荧光寿命变长
原始单层WS 2的荧光谱图如图5所示，可见等离子处理前WS 2的荧光峰是由中性激子峰和trion峰叠加而成，等离子处理后出现了新的峰，即缺陷态激子峰。
原始的和带缺陷的时间分辨荧光寿命衰减曲线如图6所示，从图6可以看出，缺陷态激子的衰减速率明显减小，荧光寿命明显变长，表明是由于缺陷使荧光寿命变长。
可见，光激发使电子从基态跃迁到激发态，电子弛豫与空穴复合，发射光子，当缺陷存在时，可以产生缺陷荧光峰，缺陷荧光寿命比固有荧光寿命长，因此造成缺陷样品的平均荧光寿命变长。
实施例3 缺陷对单层二硫化钨中激子-激子湮灭过程的影响
缺陷的存在，还会造成激子-激子湮灭(EEA)速率降低。激子-激子湮灭是一个激子将能量传递给另外一个激子的过程，属于一种非辐射过程，因此可以造成荧光寿命变短，通过数据处理可以得到激子湮灭速率。当缺陷存在时，可以束缚样品中固有激子而形成缺陷态激子，这样会造成固有激子数目减少，故用于参与激子湮灭过程的激子数减少，湮灭速率减少。
图7为不同激子对激子-激子湮灭的影响，图7中(a)给出了中性激子、trion激子和缺陷态激子的荧光谱权重作为激光功率的函数，所有荧光谱值均按峰值归一化。研究发现，中性激子峰贡献率下降，其他的与中性激子峰贡献率下降趋势相反，这是因为中性激子峰的减少会导致trion激子和缺陷态激子峰的增加。同时，trion激子对缺陷态激子没有显著贡献，说明只有中性激子被缺陷束缚。本文忽略了trion对激子-激子湮灭的影响。这也可以通过测量不同激发强度下的时间分辨荧光寿命衰减曲线来证实。随着激发强度的增加，无论是原始的还是有缺陷的单层，中性激子的寿命重量都大于99％，这比图7中(c)中trion的寿命重量(低于1％)对时间分辨荧光寿命衰减曲线的贡献更大。对于激子寿命，与中性激子的单调减少不同，trion的寿命在图7中(b)中是波动的，因此我们可以忽略trion 对EEA的影响。
与此同时，图7中(d)中原始样品和缺陷样品的平均寿命均随激发强度的增大而减小，说明非线性衰减通道EEA在高激发强度下成为主导弛豫通道，与辐射重组通道竞争，导致激子寿命缩短。此外，缺陷样品的荧光寿命长于原始样品，说明缺陷在某种程度上导致了EEA的减少。在图7中(e)中，绘制了中性激子峰值强度对激光功率的依赖关系，在低激光功率下，原始单层和缺陷单层的综合荧光强度是封闭的，说明缺陷对中性激子动力学的影响很小。但在高激光功率下，由于量子约束效应和单层二硫化钨中的强库仑相互作用，EEA变得非常显著。原始单层WS 2在高激光功率下的荧光强度高于缺陷样品，说明缺陷可以束缚中性激子，对缺陷单层WS 2的EEA过程有显著影响。
图8为不同激发强度下中性和缺陷态激子的荧光寿命成像图，如图8中(a)、(b)、(c)所示，我们发现高激发强度下的激子寿命比低激发强度下的激子寿命短，图8中(c)中缺陷态激子也观察到同样的结果，这说明在高激发强度下，EEA发生在单层WS 2中。此外，我们对不同激发强度下的时间分辨荧光寿命衰减曲线(图9-图11)进行了归一化处理，得到图12-图14，表明荧光寿命动力学强烈依赖于不同初始激发密度n(0)对应的激发强度。图12-图14使用公式(1)对中性激子和缺陷态激子进行TRPL曲线线性化数据，实线为线性拟合。
Figure PCTCN2019121186-appb-000001
其中n(t)是激子群，n 0是初始激子密度，t是衰变时间。k 0＝1/τ 0是内在激子重组率，τ 0是在没有激子-激子湮灭的低激发强度下中性激子的PL寿命或缺陷束缚。γ是激子湮灭率常数，假设与衰减时间无关。
对于单层WS 2，利用3.5％的吸收系数进行激子密度估计，并计算出每个脉冲的能量。我们假设初始激子密度与中性激子有关，每个光子都能激发一个激子。随着密度的增加，衰减趋势迅速发展，其中下面的初始激子密度的衰减信号具有相对缓慢的弛豫动力学。根据以往的研究，当激子密度超过1010cm -2时，可以触发单层WS 2中的EEA。随着密度的不断增加，激子-激子湮灭(EEA)控制着导致更快衰减的激子动力学，这是一个 额外的重要的非辐射松弛通道。
单层原始和缺陷二硫化钨中可能的激子动力学和EEA过程如图15所示：当样品在低激发强度下被激发时，主要的激子弛豫通道是激子辐射重组，EEA可以忽略不计。在原始样品的高激发强度下，EEA变得非常显著，它是一个能量从一个激子转移到另一个激子的散射过程，然后被激发到一个高能量状态，然后通过电子-声子相互作用放松到低能量状态，激发态通过非辐射弛豫途径松弛到基态，随着密度的增加，随着激发强度的增加，PL衰减速率加快，激发态衰减速度加快。引入了缺陷之后，一些激发的中性激子是由缺陷捕获，由于引入了缺陷可以束缚激子，用于发生EEA的激子数量减少，因此导致EEA速率降低，这种情况类似于一维有机金属卤化物纳米管中的激子自捕获。此外，EEA速率与激子扩散有关，缺陷可以抑制激子扩散。此外，缺陷态激子也促进辐射弛豫，成为与激子湮灭过程竞争的能量耗散途径。
在缺陷单分子层二硫化钨中，缺陷态激子的EEA速率小于中性激子。由于PL量子效率较低，缺陷态激子的数量较中性激子少。另一方面，缺陷态激子的激子结合能小于单层中中性激子的结合能，这导致了更多的非定域性和更快的激子扩散。激子扩散常数与扩散长度成正比，缺陷态激子扩散长度越长，当两个激子相互靠近时，发生EEA的时间越长。
尽管已用具体实施例来说明和描述了本申请，然而应意识到，在不背离本申请的精神和范围的情况下可作出许多其它的更改和修改。因此，这意味着在所附权利要求中包括属于本申请范围内的所有这些变化和修改。

Claims (10)

  1. 一种表征二维材料缺陷的方法，其特征在于，包括以下步骤：
    (a)提供无缺陷的二维材料衬底样品和待测二维材料衬底样品；
    (b)在同一激发波长下分别独立地对无缺陷的二维材料衬底样品和待测二维材料衬底样品进行荧光寿命成像，根据荧光寿命的变化判断有无缺陷：如果待测二维材料衬底样品的荧光寿命高于无缺陷的二维材料衬底样品的荧光寿命，则待测二维材料衬底样品为有缺陷样品；如果待测二维材料衬底样品的荧光寿命与无缺陷的二维材料衬底样品的荧光寿命相比无明显变化，则待测二维材料衬底样品为无缺陷样品。
  2. 按照权利要求1所述的表征二维材料缺陷的方法，其特征在于，步骤(b)的荧光寿命通过荧光寿命图像或荧光寿命衰减曲线获得。
  3. 按照权利要求1所述的表征二维材料缺陷的方法，其特征在于，步骤(b)还包括根据荧光寿命的变化程度判断缺陷数量：待测二维材料衬底样品的荧光寿命与无缺陷的二维材料衬底样品的荧光寿命之间的差值越大，样品的缺陷数量越多。
  4. 按照权利要求1-3任一项所述的表征二维材料缺陷的方法，其特征在于，步骤(b)中，通过荧光寿命成像系统对样品进行荧光寿命成像，包括以下步骤：
    激光器发射激光，激光通过振镜，再由分束镜反射至物镜聚焦至样品，样品产生的光信号由物镜采集后透过分束镜，再用滤波片对样品的荧光寿命进行检测；然后用光电探测器检测光信号，再利用时间相关单光子计数系统对光电探测器和激光器进行同步，通过振镜扫描得到荧光寿命图像。
  5. 按照权利要求4所述的表征二维材料缺陷的方法，其特征在于，激光器的激发波长为450-500nm，激发频率为35-45MHz；
    优选地，滤波片的波长为500-700nm；
    优选地，时间相关单光子计数系统的分辨率为6-10ps。
  6. 按照权利要求1-3任一项所述的表征二维材料缺陷的方法，其特征在于，所述二维材料包括经化学气相沉积直接生长在衬底上的二维材料，或，经机械剥离或光刻胶转移方法转移到衬底上的二维材料；
    优选地，所述二维材料包括过渡金属硫化物、过渡金属硒化物或过渡 金属碲化物，优选包括WS 2、MoS 2、ReS 2、WSe 2、MoSe 2、Bi 2Se 3、MoTe 2、WTe 2或Bi 2Te 3中的一种。
  7. 按照权利要求1-3任一项所述的表征二维材料缺陷的方法，其特征在于，所述衬底包括金属铜、镍、铂、铁或合金衬底。
  8. 按照权利要求1-3任一项所述的表征二维材料缺陷的方法，其特征在于，所述缺陷包括点缺陷、晶界线缺陷、褶皱或破损边缘中的一种或几种。
  9. 一种权利要求1-8任一项所述的表征二维材料缺陷的方法在检测基于二维材料的元件中的应用。
  10. 根据权利要求9所述的应用，其特征在于，所述基于二维材料的元件包括二极体、自旋元件、场效电晶体或穿隧电晶体。
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