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 PDFInfo
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- G—PHYSICS
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- 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
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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
- G01N21/6408—Fluorescence; Phosphorescence with measurement of decay time, time resolved fluorescence
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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
- G01N21/645—Specially adapted constructive features of fluorimeters
- G01N21/6456—Spatial resolved fluorescence measurements; Imaging
- G01N21/6458—Fluorescence microscopy
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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
- 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
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- 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
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Claims (10)
- 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。
- 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。
- 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。
- 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。
- 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。
- 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。
- 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。
- 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。
- 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。
- 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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