CN111982945A - Method and application of time-resolved dark field image acquisition based on ultrafast transmission electron microscope system - Google Patents

Abstract

A method and application for obtaining a high-contrast ultrafast time-resolved dark field image based on an ultrafast transmission electron microscope system. By using a high-convergence angle incident beam imaging, using a large-size objective lens aperture, and imaging with diffracted electrons as a signal, a time-resolved dark-field image at an ultrafast scale is obtained. Compared with the ultrafast TEM bright-field image, it has opposite contrast. The dark field image reduces the background signal. Due to the combination of the high convergence angle and the large-size objective diaphragm, and through the optimal adjustment of double-beam diffraction and dark field image, the signal-to-noise ratio of the collected signal is improved, compared with the diffraction mode. The signal can more directly observe the change process of different regions inside the sample, has high spatial resolution, expands the scope of the ultrafast electron microscope dark field method to characterize the sample, can analyze the details of the contrast change of the nanoparticles after being excited by the laser, and can be applied Characterization of lattice relaxation and resonance dynamics in materials with different lattice orientations on ultrafast timescales.

Description

Method and application of time-resolved dark field image acquisition based on ultrafast transmission electron microscope system

technical field

The present disclosure belongs to the technical field of dark field characterization of electron microscopy and ultrafast electron microscopy, and relates to a method and application for obtaining a time-resolved dark field image based on an ultrafast transmission electron microscope system, in particular to a method for obtaining a time-resolved dark field image based on an ultrafast transmission electron microscope system. Methods and applications for high-contrast ultrafast time-resolved dark field images.

Background technique

Transmission electron microscopy is an important material characterization tool. After rapid development of transmission electron microscopy technology, the spatial resolution has reached the sub-angstrom level, which has greatly satisfied the experimental needs. Characterizing the change process of materials by transmission electron microscopy has become an important development trend at present. Such as in situ transmission electron microscopy and 4D ultrafast transmission electron microscopy.

Ultrafast transmission electron microscopy is combined with ultrafast optical means, which has the advantages of high spatial resolution of transmission electron microscopy and high time resolution of ultrafast optics, and breaks through the limitation of the acquisition speed of charge-coupled device (CCD) cameras, realizing femtosecond to The dynamics of material structure changes in the microsecond range, thereby revealing atomic-scale changes in physical or chemical processes. Ultrafast transmission electron microscopy has developed a variety of characterization techniques to analyze the plasmonic effects of materials based on image or diffraction information, or in combination with electron energy loss spectroscopy (EELS).

At present, based on ultrafast TEM data acquisition, bright field images and diffraction spots are mainly used to study the internal dynamic changes of materials after being excited by pulsed lasers. In 2018, some studies were based on ultrafast TEM bright field images. Contrast change, the lattice vibration process generated by pumping inside the plasmonic gold nanoparticles and between the particles is studied, but because the bright field image is mainly transmitted electrons, the lattice change leads to the signal of the contrast change of the bright field image, Compared with the transmission electron signal, it only accounts for a small part, so there is a problem that the contrast change is not obvious. At the same time, the number of imaging electrons in the ultrafast transmission electron microscope is lower than that of the conventional transmission electron microscope, so the signal-to-noise ratio of the result obtained is relatively poor, and the final image is The lattice change process of the sample after laser excitation cannot be clearly revealed. The bright-field image pattern captured by the larger electron beam spot and the smallest size of the objective lens in the optoelectronic mode is shown in Figure 1.

In the dark field image, diffracted electrons are used for imaging. Compared with the bright field image, the contrast is opposite, the sample area becomes a bright area, the substrate area becomes a dark area, the background signal is weakened, the change of the sample signal intensity is more obvious, and the signal-to-noise ratio is Compared with the bright field image, it has been greatly improved, and compared with the bright field image of the ultrafast system, it can more intuitively represent the change of the sample in the ultrafast process.

In conventional transmission electron microscopes, the shooting of dark field images often requires the electron beam to be irradiated on the sample in an approximately parallel state, and the convergence angle is relatively small, so that the dark field image can be shot with the smallest size of the objective lens to ensure that the diffracted electrons pass through and block the Most of the electrons are scattered (inelastically), but these methods are not directly applicable to ultrafast transmission electron microscopy. In ultrafast transmission electron microscopy, electrons are excited by laser pulses in the form of pulses, and the number of electrons generated is much less than that of conventional electron microscopy, so the dark field image contrast is very poor, and in optoelectronic mode, a larger electron beam spot and the smallest size of the objective lens aperture are used. The dark field image spectrum obtained by shooting is shown in Figure 2.

At present, the dark-field image mode based on ultrafast transmission electron microscopy is mainly based on the mechanical motion of the observed sample, and the application range of the work based on dark-field images is small, and there are great restrictions on the selection of samples, and small-sized samples cannot be applied, such as Characterization of samples such as nanoparticles. In addition, there are very few existing studies that use dark field images to characterize large-size samples, and the image contrast obtained is poor. The degree of change process cannot be applied to accurate kinetic analysis. Therefore, there is still a need for an intuitive and high signal-to-noise method to analyze the dynamic changes of samples on ultrafast time scales.

SUMMARY OF THE INVENTION

(1) Technical problems to be solved

The present disclosure provides a method and application for obtaining a time-resolved dark field image based on an ultrafast transmission electron microscope system, so as to at least partially solve the above-mentioned technical problems.

(2) Technical solutions

According to an aspect of the present disclosure, there is provided a method for obtaining a time-resolved dark field image based on an ultrafast transmission electron microscope system, comprising: step S12: in the thermionic mode, adjusting to obtain approximate double-beam diffraction of the sample and adopting an objective lens The diaphragm covers the diffraction spot to obtain a dark field image; Step S13: in the photoelectron mode, the electron beam spot is converged at a larger convergence angle, and the focused beam diffraction of the sample is obtained in the diffraction mode; in the dark field mode, the incident based on the thermionic mode The tilt angle of the electron beam, using the objective diaphragm to select the diffraction spot, obtains the time-resolved dark field image of the sample in the photoelectron mode, where the larger convergence angle makes the diffraction spot area in the photoelectron mode larger than that in the thermionic mode .

Wherein, the objective lens diaphragm is a large-sized objective lens diaphragm, and the diaphragm aperture of the large-sized objective lens diaphragm is between 10 μm and 100 μm; preferably between 20 μm and 40 μm.

Wherein, before step S12, it further includes: step S11: in the ultrafast transmission electron microscope system, adjust the focus position of the pump laser to focus on the sample position, and focus the detection laser on the filament to generate detection electron pulses.

In some embodiments, step S11 includes: before the pump laser enters the sample chamber of the TEM system, adding a visible light mirror to reflect the pump laser to the beam analyzer, and adjusting the position of the beam analyzer to find the pump laser focus position and focus According to the size of the spot, the actual distance between the focus point and the focus lens and the size of the focus spot are obtained; according to the focus position, the position of the focus lens in the pump light path is adjusted by the three-dimensional displacement stage; the carbon film is placed in the ultrafast transmission electron microscope, and the minimum energy pump light is used Excite the position of the carbon film to achieve the coincidence of the excitation position of the pump laser with the position of the photogenerated detection electrons; and finely adjust the position of the focusing lens to find the best focusing position.

In some embodiments, after step S13, it further includes: step S14: using an automatic acquisition module to acquire images, the automatic acquisition module realizes its function through software, or hardware, or a combination of hardware and software. In step S14, the time-resolved dark field image can be automatically collected and saved by controlling the one-dimensional electric displacement stage and the imaging device of the transmission electron microscope system; or the pump laser can be controlled by controlling the mechanical shutter switch in the pump laser light path, and the reference image and the excitation image can be collected. .

In some embodiments, after step S14, the method further includes: batch processing the collected images.

In some embodiments, the batch processing of the collected images includes one or more of image averaging as a reference image, image offset calibration, image intensity analysis, and result output, etc.; wherein the image intensity analysis includes: a single pixel One or more of intensity analysis, area intensity analysis, and line intensity analysis; wherein the result output includes: image noise processing output, intensity curve output, position calibration output, image cross-correlation value output, image drift amount output, Fourier analysis One or more of output and video output.

According to another aspect of the present disclosure, there is also provided a time-resolved dark field image obtained by the method described in the present disclosure for analyzing the details of the contrast change of the nanoparticle after being excited by the laser, and characterizing the difference in the ultrafast time scale. Application of lattice relaxation and resonance dynamics processes to lattice orientation of materials.

(3) Beneficial effects

As can be seen from the above technical solutions, the method and application for obtaining a time-resolved dark field image based on an ultrafast transmission electron microscope system provided by the present disclosure have the following beneficial effects:

1. By using a high-convergence angle incident beam imaging, using a large-size objective lens aperture, and imaging with diffracted electrons as a signal, a time-resolved dark-field image at an ultrafast scale can be obtained. Compared with the ultrafast transmission electron microscope bright-field image, it has the opposite The contrast dark field image reduces the background signal. Due to the combination of the high convergence angle and the large size of the objective lens, and through the optimal adjustment of double beam diffraction and dark field image, the signal-to-noise ratio of the collected signal is improved. Compared with diffraction The mode signal can more directly observe the change process of different regions inside the sample, has high spatial resolution, expands the scope of the ultrafast electron microscope dark field method to characterize the sample, and can analyze the details of the contrast change of the particles after being excited by the laser. The time-resolved dark field image obtained by the present disclosure has higher contrast and more obvious lattice changes, and can observe the dynamic change process of the sample in ultrafast time scales, such as picoseconds, nanoseconds, or microseconds. It contains the interaction information of electrons and phonons and the tilting information of the lattice direction, as well as the heat scattering electrons, which can be used for the analysis of heat scattering electron signals.

2. The image acquisition process of the present disclosure adopts automatic acquisition, and different reference modes can be selected to ensure that the acquisition process reduces external interference and improves acquisition efficiency.

3. The present disclosure also proposes a corresponding data processing and analysis program, which can analyze the collected data images in batches, perform drift correction on the sample position, perform noise removal and filtering processing on the image, output single pixel intensity analysis, regional pixel intensity analysis, Line strength analysis, output video, etc.

Description of drawings

Fig. 1 is a bright-field image spectrum obtained by taking a larger electron beam spot and a minimum size objective lens aperture in the optoelectronic mode in the prior art.

FIG. 2 is a dark field image pattern obtained by taking a larger electron beam spot and a minimum size objective lens aperture in the optoelectronic mode in the prior art.

3 is a schematic structural diagram of an ultrafast transmission electron microscope system according to an embodiment of the present disclosure.

FIG. 4 is a dark field image map obtained by using a high convergence angle and a large-sized objective lens aperture based on the method of the present disclosure.

5 is a flowchart of a method for acquiring a time-resolved dark field image based on an ultrafast transmission electron microscope system according to an embodiment of the present disclosure.

6A-6C are schematic diagrams of determining a focus position according to a carbon film ablation area according to an embodiment of the present disclosure.

7A-7D are schematic diagrams illustrating approximate double-beam diffraction and dark-field images of a sample obtained by adjusting in a thermionic mode according to an embodiment of the present disclosure.

8A-8D are schematic diagrams of adjusting a time-resolved dark field image of a sample in an optoelectronic mode according to an embodiment of the present disclosure.

FIG. 9 is a flowchart of image acquisition and storage performed by an automatic acquisition module for acquiring negative-time reference images according to an embodiment of the present disclosure.

FIG. 10 is a flowchart of image acquisition and storage performed by an automatic image acquisition module controlling a mechanical shutter according to an embodiment of the present disclosure.

FIG. 11 is a changing process of a dark field image of a collected sample according to an embodiment of the present disclosure.

FIG. 12 is a standard deviation analysis result of a time-resolved dark field image according to an embodiment of the present disclosure.

FIG. 13 is an intensity change curve of a certain area of a time-resolved dark field image according to an embodiment of the present disclosure.

【Symbol Description】

01-laser; 02-one-dimensional electric displacement stage;

03-mechanical shutter; 04-filament;

05-sample; 06-ultrafast TEM/EM cavity;

07 - Imaging device.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present disclosure more clear, the present disclosure will be further described in detail below with reference to the specific embodiments and the accompanying drawings. "Between" contains endpoint values.

In conventional TEM, the sample is in a stable state and the temperature remains stable. Dark field images mainly study the inhomogeneity inside the sample, providing a static characterization of the sample structure, such as the location of internal defects, the size of grains or the enrichment of different regions The difference between particles, etc., so often observed in a conventional electron microscope is a non-uniform sample. The time-resolved dark-field image obtained based on the present disclosure has high contrast, high signal-to-noise ratio and high spatial resolution, and has become a means of studying dynamic signals of structural changes in a specific area, such as studying the phonon in a sample. There is no requirement on whether the sample is homogeneous or not. Therefore, the time-resolved dark field image in the present disclosure represents a different meaning and process from the conventional electron microscope dark field image.

In this disclosure, the terms "approximately" and "approximately" mean equivalents within the allowable error range due to limitations of the instrument itself or limitations of experimental operations.

first embodiment

In a first exemplary embodiment of the present disclosure, a method for acquiring a time-resolved dark field image based on an ultrafast transmission electron microscope system is provided.

The ultrafast transmission electron microscope system suitable for this method is introduced first.

3 is a schematic structural diagram of an ultrafast transmission electron microscope system according to an embodiment of the present disclosure.

Referring to FIG. 3 , the ultrafast transmission electron microscope system of the present disclosure includes: a laser system for generating an ultrashort pulse laser, which is divided into two parts, a detection laser and a pump laser after a beam splitter; called electron microscope system), including electron microscope cavity 06, filament 04, electron microscope imaging device 07; and one-dimensional electric displacement stage 02, three-dimensional displacement stage and mechanical shutter 03 arranged in the optical path between the laser system and the transmission electron microscope system; the transmission electron microscope system The electron microscope system can work in the thermionic mode. The electron beam is generated by heating the filament in the electron microscope cavity. The electricity can introduce the detection laser into the electron gun to generate photo-generated pulsed electrons. The photo-generated pulsed electrons are accelerated through the sample area and finally imaged in the electron microscope. The device; the pump laser can also be introduced into the sample area of the electron microscope to excite the sample to produce transient lattice morphology changes; wherein, the three-dimensional displacement stage adjusts the position of the focusing lens to focus the incident probe laser and pump laser on the filament area and the sample area, respectively ; One-dimensional motorized stage, to control the optical path of the pump laser; Mechanical shutter, to control the switch of the pump laser.

In this embodiment, as shown in FIG. 3 , the ultrafast transmission electron microscope system includes: a laser 01, a one-dimensional electric displacement stage 02, a mechanical shutter 03, an ultrafast transmission electron microscope 06, and a three-dimensional displacement stage, wherein the ultrafast transmission electron microscope 06 It includes a filament 04 and an imaging device 07, wherein the filament 04 is used to generate a detection electron pulse, and a sample 05 is placed in the area where the detection electron pulse is accelerated. A beam of laser is used as the detection light source, which is called detection laser. After frequency doubling conversion, it is finally focused on the filament 04 in the ultrafast transmission electron microscope 06, and a detection electron pulse is generated, and the detection electron pulse is accelerated through the cavity of the ultrafast transmission electron microscope 06. , penetrates through the sample 05; another laser is used as the excitation light source, called the pump laser, passes through the mechanical shutter 03, and finally focuses on the sample 05 in the ultrafast transmission electron microscope 06. The filament 04, the sample 05 and the imaging device 07 are located on the same straight line, and the detection electron pulses emitted by the filament penetrate through the sample 05 and then image on the imaging device 07 of the ultrafast transmission electron microscope 06.

In this embodiment, the ultrafast transmission electron microscope 06 can generate the thermionic beam current by heating the filament 04 , or can generate probe electrons by introducing laser light to illuminate the area of the filament 04 .

In this embodiment, the three-dimensional displacement stage is used to adjust the position of the focusing lens at the entrance of the sample chamber of the ultrafast transmission electron microscope system, and to adjust the position where the pump laser is focused on the sample 05 and the size of the focusing area.

FIG. 4 is a dark field image map obtained by using a high convergence angle and a large-sized objective lens aperture based on the method of the present disclosure. 5 is a flowchart of a method for acquiring a time-resolved dark field image based on an ultrafast transmission electron microscope system according to an embodiment of the present disclosure.

Referring to FIG. 4 , the time-resolved dark-field image obtained by this method can characterize small-sized nanoparticles and expand the application range of the time-resolved dark-field image.

The following describes a method for obtaining a time-resolved dark field image based on the ultrafast transmission electron microscope system of the present disclosure in conjunction with the ultrafast transmission electron microscope system of the present disclosure. Referring to FIG. 3 and FIG. 5 , the method includes:

Step S11: in the ultrafast transmission electron microscope system, adjust the focus position of the pump laser to focus on the sample position, and focus the detection laser on the filament to generate a detection electron pulse;

In this embodiment, a preferred method for adjusting the focus position of the pump laser is proposed based on the structure of the ultrafast transmission electron microscope system.

The adjustment methods of the focused laser include:

Step 1: Before the pump laser enters the sample chamber of the TEM system, add a visible light reflector to reflect the pump laser to the beam analyzer, adjust the position of the beam analyzer to find the pump laser focus position and focus spot size, and obtain the focus point The actual distance from the focusing lens and the size of the focusing spot;

Step 2, according to the focus position, adjust the position of the focus lens in the pump light path through the three-dimensional displacement stage 02;

Step 3, put a carbon film (for example, a 50-mesh carbon film) in the ultrafast transmission electron microscope 06, and use the minimum energy pump light to excite the position of the carbon film, so that the excitation position of the pump laser is coincident with the position of the photogenerated detection electrons;

Step 4: Finely adjust the position of the focus lens back and forth to find the best focus position.

6A-6C are schematic diagrams of determining a focus position according to a carbon film ablation area according to an embodiment of the present disclosure.

When the position of the carbon film is in front of the focus of the focusing lens, the ablation area is shown in Figure 6A, when the position of the carbon film is behind the focus of the focus lens, the ablation area is shown in Figure 6B, when the position of the carbon film is at the focus of the focus lens , the ablation area is shown in Fig. 6C, which is approximately a perfect circle.

Specifically, the external optical path of the ultrafast transmission electron microscope is precisely adjusted based on the following operation steps so that the pump laser and the probe laser can be focused on the exact position in the ultrafast transmission electron microscope, that is, the pump laser and the probe laser are both focused on the sample, and can achieve Control of focus area. Step 1, put a 50-mesh carbon film into the electron microscope system as a sample, find small particles as a reference in thermionic mode, and adjust the particles to appear on the electron microscope imaging device 07 under a suitable multiple; step 2, adjust the ultrafast transmission electron microscope. When the magnification of the system is up to 50 times, the position of the particles under this magnification can be obtained; in step 3, a low-contrast area can be ablated on the carbon film by using a low-power pump laser, and the focus area of the pump laser on the carbon film can be adjusted. The small particles are located in the center of the low-contrast area, and the focus position of the pump laser is overlapped with the detection area of the detection electron; step 4, adjust the three-dimensional stage to move the focus lens forward and backward to change the focus position, and obtain the optimal focus according to the shape of the final ablation area lens position.

In some embodiments, chemically synthesized gold triangular nanoprisms are selected as samples, and graphene copper meshes are selected as substrates, and the method based on the present disclosure can be used to characterize the lattice dynamics of gold nanosheets.

Step S12: in the thermionic mode, adjust to obtain approximate double-beam diffraction and dark field images of the sample;

7A-7D are schematic diagrams illustrating approximate double-beam diffraction and dark-field images obtained by adjusting the sample in thermionic mode according to an embodiment of the present disclosure, wherein FIG. 7A is a bright-field image of the sample under thermionic mode. Figure 7B is the approximate double-beam diffraction pattern obtained by adjusting the sample in thermionic mode; Figure 7C is the spectrum after selecting the objective lens aperture to cover the diffraction spot; Figure 7D is the final dark field image obtained in thermionic mode.

7A-7D of the accompanying drawings, in this embodiment, the method for adjusting the orientation (with axis) of the electron microscope sample to obtain a dark field image in the thermionic mode includes the following sub-steps:

Sub-step S12a: select a suitable sample area in thermionic mode;

In this embodiment, the filament 04 in the ultrafast transmission electron microscope 06 is heated to the thermal electron beam state, and a suitable sample area is selected in the thermal electron mode, for example, as shown in FIG. 7A, a suitable particle sample is found, and the pump laser is introduced into the transmission Electron microscope system sample 05 area.

Sub-step S12b: Use the selective aperture to cover the sample, enter the diffraction mode, adjust the diffraction spot to the center of the screen, adjust the tilt angle of the sample to make the diffraction pattern approximate to double-beam diffraction under the electron microscope system, and ensure that there is basically no difference between the diffracted beam and the transmitted beam. interaction;

In this embodiment, a selective aperture is used to cover the sample, enter the diffraction mode, adjust the diffraction spot to the center of the screen, and adjust the tilt angle of the sample, so that the sample is diffracted in the diffraction mode close to the double-beam diffraction mode, as shown in Figure 7B. The sample is the diffraction spot pattern obtained corresponding to the chemically synthesized gold triangular nanoprism, and the diffraction spot circled after calibration corresponds to the Au(220) crystal plane.

It is particularly emphasized that, in the present disclosure, the diffraction mode needs to be a double-beam diffraction mode, which is different from the operation of conventional electron microscope dark field images. In the conventional electron microscope dark-field image, it is enough to obtain the diffraction spots of the sample, and it is not necessary to use the double-beam diffraction mode.

Sub-step S12c: enter the dark field mode, adjust the incident angle of the incident electron beam so that the corresponding diffraction spot is located at the position of the original transmission spot, and the system will record the current tilt angle of the incident electron beam;

In this sub-step S12c, the recorded tilt angle of the current incident electron beam will be used in the subsequent step S13.

Sub-step S12d: selecting the objective lens aperture to cover the diffraction spot, and exiting the diffraction mode to obtain a dark field image;

In the present disclosure, a large-sized objective lens diaphragm is adopted, and the minimum size of the diaphragm aperture of the selected objective lens diaphragm is larger than or equal to the size of the objective lens diaphragm used in the prior art, and the diaphragm aperture is between 10 microns and 100 microns. For example, in one example, an objective diaphragm with a physical size of 20 μm can be used, corresponding to a collection half angle of about 7.6 mrad, as shown in FIG. 7C . Preferably, the diameter of the objective lens aperture (aperture aperture) is greater than or equal to 20 μm and less than or equal to 40 μm. For example, in a specific example, the aperture of the 2# objective lens aperture is 20 μm, and the 3# objective lens aperture aperture is 40 μm.

In conventional electron microscopes, due to the high intensity of the electron beam, the light spot is evenly spread out to ensure that the light is approximately parallel to the sample. When using the objective lens aperture to select the diffraction spot, the minimum size objective lens aperture is used to select the diffraction spot to ensure the least heat scattering. Electrons pass through. In some examples of the present disclosure, the electron pulse is laterally broadened from the filament to the sample position due to the space charge effect, the coherence is weakened, and the edges of the formed diffraction spots are blurred relative to the thermionic mode. The incident electron beam irradiates the sample with an approximate convergent beam. The larger the incident electron beam convergence angle, the larger the diffraction spot area obtained in the reciprocal space, so the diffraction spot area obtained by the photo-generated electron mode is larger than that of the conventional electron microscope. Based on the above factors, the objective lens The size of the aperture should be large enough to ensure that most of the diffracted signal electrons pass through, so as to ensure that the time-resolved dark field image has sufficient signal intensity. In the reciprocal space, the large-sized objective diaphragm will contain more scattered electrons at an angle. Therefore, based on the amorphous region in the dark-field image, the change of the heat-scattered electron signal can be detected. Therefore, the dark-field image can not only study the area of the crystalline sample The diffraction contrast changes, and the scattering changes in the amorphous region can be studied, which contains more sample change information than the traditional electron microscope dark field image.

Sub-step S12e: Fine-tune the tilt angle of the sample according to the dark field image, so that the region of interest is a bright region.

The above sub-steps S12b to S12e are performed alternately until the target area of the dark field image is bright, and the diffraction spot is in a double beam diffraction pattern, which is an ideal dark field image. The result is shown in FIG. 7D .

In the process of switching from the thermionic mode to the optoelectronic mode, the mode switching can be performed by the following methods:

Reduce the heating current of the filament 04 of the TEM system to a state of no thermal electron emission, and the imaging device 07 based on the TEM system should not be able to detect electrons;

Introduce the detection laser into the filament 04 area of the TEM system, focus it on the center of the filament surface, use photoelectron pulse imaging to find the sample area, and readjust the TEM system to obtain a bright field image with no obvious aberration under the photoelectron pulse of the sample.

Step S13: in the photoelectron mode, the electron beam spot is converged with a larger convergence angle, and the convergent beam diffraction of the sample is obtained in the diffraction mode; in the dark field mode, based on the tilt angle of the incident electron beam in the thermionic mode, the objective aperture is used Select the diffraction spot to obtain a time-resolved dark field image of the sample in the photoelectron mode, wherein the larger convergence angle makes the diffraction spot area in the photoelectron mode larger than the diffraction spot area in the thermionic mode;

The objective lens aperture of the diffraction spot here is the same size of the objective lens aperture in step S12, and both are large-sized objective lens apertures.

In some examples of the present disclosure, the number of photogenerated electrons is less than that of a conventional electron microscope. In order to improve the brightness, the photogenerated electron beam is converged on the sample in an approximate convergent beam mode to obtain approximate convergent beam diffraction of the sample, and the objective lens aperture is selected to cover the diffraction spot, The larger the size of the objective lens aperture is, the more heat scattered electrons are transmitted, and the dark field image will contain more heat scattered electrons from different angles. Therefore, the time-resolved dark field image based on the present disclosure can be used for the study of thermally scattered electrons at different angles.

Therefore, in this operation, due to the electron Coulomb repulsion of electrons from the electron gun to the sample, the electron beam expands and the diffraction spot expands laterally. At the same time, the number of electrons in the photoelectron mode is relatively small compared to the thermal emission mode, in order to generate enough signal The intensity of the incident electrons needs to be concentrated on the gold particles with a large convergence angle, and the diffraction spot area is larger than that of the thermionic mode. Based on the above factors, when characterizing nanoparticle materials such as gold nanoparticles, it is necessary to use a large-sized objective lens aperture to ensure that most of the diffracted electrons pass through the objective lens aperture, and finally obtain a time-resolved dark field image with sufficient signal intensity.

8A-8D are schematic diagrams of adjusting a time-resolved dark-field image of a sample in an optoelectronic mode according to an embodiment of the present disclosure, wherein FIG. 8A is a bright-field image spectrum obtained by converging an incident beam in an optoelectronic mode; 8B is the approximate convergent beam diffraction pattern obtained in the photoelectron mode; FIG. 8C is the approximate convergent beam diffraction spot obtained in the photoelectron mode with the objective lens aperture; FIG. 8D is the dark field image spectrum obtained in the photoelectron mode.

In some examples, the method of obtaining a time-resolved dark field image in photogenerated electron mode includes the following sub-steps:

Sub-step S13a: in the optoelectronic mode, converging the photogenerated detection electron pulses on the sample, close to the converging beam mode, and the bright-field image spectrum obtained by converging the incident beam is shown in FIG. 8A;

Sub-step S13b: enter the diffraction mode, and obtain the approximate convergent beam diffraction of the sample region under the photogenerated detection electron pulse;

The approximate convergent beam diffraction pattern obtained in the photoelectron mode is shown in FIG. 8B , and the convergence angle can be calculated from the diffraction spots. For example, in this embodiment, the convergence angle calculated from the diffraction spots is about 9.9 mrad.

Sub-step S13c: enter the dark field mode, adjust the tilt angle of the electron beam according to the inclination angle of the incident electron beam recorded in the thermionic mode, move the corresponding diffraction spot to the original transmission spot position, and adjust the spot position to make the diffraction spot the highest brightness;

Sub-step S13d: select a large-sized objective lens aperture to cover the central diffraction spot, and the size of the objective lens aperture should be large enough to ensure that most of the diffracted electrons pass through the objective lens aperture, block the transmitted beam electrons, exit the diffraction mode, and obtain a dark field image;

In this sub-step S13d, the approximate convergent beam diffraction spot pattern in the photoelectron mode is shown in Fig. 8C. The size of the objective aperture is large enough to ensure that most of the diffracted electrons pass through the objective aperture and block the transmitted beam electronic.

Sub-step S13e: adjust the position of the electron beam, adjust the size of the electron beam spot, and obtain the best dark field image;

The dark-field image pattern finally obtained in the optoelectronic mode is shown in Fig. 8D.

Step S14: using an automatic acquisition module to acquire images;

FIG. 9 is a flowchart of image acquisition and storage performed by an automatic acquisition module for acquiring negative-time reference images according to an embodiment of the present disclosure. The image acquisition process will be described below with reference to FIG. 9 .

In some embodiments, the automatic acquisition module can control the one-dimensional motorized stage 02 and the TEM system imaging device 07 to automatically acquire and save time-resolved dark field images. As shown in Figure 9, use the electron microscope system imaging device control software Gatan Digital Micrograph software to write a script to control the movement of the one-dimensional stage and the imaging device to collect images. The automatic image acquisition process is as follows:

Sub-step S141a: control the ultrafast transmission electron microscope system according to the parameters set by the user. The parameters of the automatic acquisition program set by the user include one or more of the following parameters: the starting position, final position and stepping distance of the one-dimensional motorized stage, etc., and the transmission electron microscope The exposure time of the system imaging device, and the pixel value of the acquired image, etc.

Sub-step S141b: the automatic acquisition module controls the one-dimensional electric displacement stage to move to a specified position, adjusts the optical path of the pump laser, and controls the interval time between the pump laser pulse reaching the sample and the detection electron pulse reaching the sample 05;

Sub-step S141c: the automatic acquisition module acquires the image according to the parameters, assigns an ID to the image, and saves the image to a local file;

Sub-step S141d: determine whether to collect a reference image, if yes, execute sub-step S141e, otherwise execute sub-step S141g;

Sub-step S141e: control the one-dimensional electric displacement stage to move from the current position P (the original position as the reference) to the negative time position b, collect and save the reference image;

Sub-step S141f: control the one-dimensional electric displacement stage to return to the original position P, re-collect the image and save it;

Sub-step S141g: the system judges whether the acquisition process is completed, and if so, ends the acquisition process, otherwise controls the one-dimensional electric displacement stage to the next position to acquire images;

Sub-step S141b to sub-step S141g are repeated, and finally the acquisition process of all dark field images is completed.

FIG. 10 is a flowchart of image acquisition and storage performed by an automatic image acquisition module controlling a mechanical shutter according to an embodiment of the present disclosure.

In other embodiments, at each position of the one-dimensional stage, the automatic acquisition module can control the mechanical shutter switch in the pump laser light path to control the pump laser, and collect the reference image and the excitation image.

Referring to Figure 10, at each position of the one-dimensional stage, the automatic acquisition module can control the mechanical shutter switch in the pump laser light path to control the pump laser. The process of collecting the reference image and the excitation image is as follows:

Sub-step S142a: control the imaging system of the transmission electron microscope system according to the parameters set by the user, and the parameters of the automatic acquisition module set by the user are the same as the settings of the automatic acquisition module in the above-mentioned sub-step S141a.

Sub-step S142b: the automatic acquisition module controls the mechanical shutter to close, the pump laser will not be introduced into the sample area of the TEM system, and the sample is in an unexcited state;

Sub-step S142c: the automatic acquisition module controls the one-dimensional displacement stage to move to a designated position;

Sub-step S142d: the automatic acquisition module acquires a reference image according to the parameters, assigns an ID to the image, and saves the image in a local folder;

Sub-step S142e: the automatic acquisition module controls the opening of the mechanical shutter, the pump laser is introduced into the sample area of the TEM system, and the sample is excited by the laser to produce an instantaneous change;

Step S142f: the automatic acquisition module acquires the sample excitation image according to the parameters, assigns an ID to the image and saves it to another local folder;

Sub-step S142g: the system judges whether the acquisition process is completed, and if so, ends the acquisition process, otherwise controls the one-dimensional electric displacement stage to the next position to acquire images.

Sub-steps S142b to S142g are repeated, and the sample state when the sample is excited by the pump laser pulse and the sample state when it is not excited are taken as reference signals at different intervals to complete the entire image acquisition process.

The above automatic collection module is described as being implemented by software, but the present disclosure is not limited thereto. In other embodiments, the same function may be implemented by different software, or hardware, or a combination of hardware and software.

Second Embodiment

In a second exemplary embodiment of the present disclosure, a method for processing and analyzing a time-resolved dark field image is provided.

For example, batch processing can also be performed on the obtained dark field images. In some examples, batch processing is performed on each image, including one or more of: image sorting, denoising, image averaging as a reference image, image offset calibration, image intensity analysis, and result output, among others. Image intensity analysis, including one or more of single-pixel intensity analysis, area intensity analysis, and line intensity analysis, and the result output includes: image noise processing output, intensity curve output, position calibration output, image cross-correlation value output, and image drift One or more of output, Fourier analysis output, and video output.

FIG. 11 is a changing process of a dark field image of a collected sample according to an embodiment of the present disclosure.

Using a setting program, the sample changes can be visually displayed through video, and the results are shown in Figure 11, which show the dark field image outputs corresponding to zero time, 170 picoseconds (ps), 230 ps and 370 ps, respectively. Video outputs include the following:

The reference image A is used as the reference image B after denoising, the original image is calibrated with the reference image B after denoising, and the video is output;

The reference image A is denoised and processed by median filtering as reference image C, the original image is denoised and calibrated with reference image C, and is processed by median filtering to output video;

The reference image A is used as the reference image B after the noise is removed, the original image is calibrated with the reference image B after the noise is removed, and the image position is calibrated with the reference image B after difference processing, and output video;

The reference image A is denoised and processed by median filter as reference image C. The original image is de-noised and calibrated with reference image C. After the image position is calibrated, it is processed by median filter and difference with reference image C to output video.

The image intensity output can select one or more pixel points in the image, compare the pixel point value changes under different delays, and perform Fourier transform on the pixel point value change curve to obtain the intensity change frequency distribution. Or select pixels and output curves in a certain area around a certain position of the image, or rotate the image to output multi-pixel superimposed line intensity at a certain position of the image, mark the position of the line intensity analysis area in the image, and output the two-dimensional result of the line intensity and 3D results. The output of the cross-correlation value includes the change curve of the maximum cross-correlation value between the image and the reference image B after the noise is removed, and the change curve of the maximum cross-correlation value with the reference image B after the image is filtered after the noise is removed.

The formula for calculating the cross-correlation value is as follows:

where the contrast C _x,y (t) is defined as:

为延时时间t时图像的平均像素强度。where time t represents the laser pulse delay t when the one-dimensional stage is at the specified position, t' represents the time of the selected reference image, and I _{x, y} (t) is the position (x, y) in the image. The delay time is t pixel intensity, while

is the average pixel intensity of the image at the delay time t.

FIG. 12 is a standard deviation analysis result of a time-resolved dark field image according to an embodiment of the present disclosure.

Quantitative analysis was performed on all pictures based on the output video of the analysis program of the present disclosure. The standard deviation analysis can obtain the change of the data dispersion. The obtained series of images are analyzed to obtain the area with the most significant pixel intensity change in the particle area, and the standard deviation analysis of each pixel intensity of the image is performed, and the result is shown in Figure 12. The brightness in the figure represents the intensity of the intensity change. Based on this, select one of the corner areas to observe the intensity change process. FIG. 13 is an intensity change curve of a certain area of a time-resolved dark field image according to an embodiment of the present disclosure. As shown in FIG. 13 , the abscissa is the delay time of the pump laser pulse relative to the detection electron pulse, and the ordinate is the dark field Like the intensity average of the selected region, a clear intensity oscillation process can be observed, which is due to the pump laser excitation of lattice oscillations in the sample, including the phonon coupling between the gold particles and the graphene substrate, resulting in a frequency of High frequency oscillation of G Hz, the lattice orientation changes, resulting in changes in the intensity of the dark field.

Third Embodiment

In a third exemplary embodiment of the present disclosure, an application of the above method for acquiring a time-resolved dark field image based on an ultrafast transmission electron microscope system is provided.

In some embodiments, the method can be applied to lattice dynamics.

The method of the present disclosure mainly reflects a single diffraction contrast. Nanoparticles (such as gold particles) are excited by a pump laser. After electrons in the sample gain energy in the femtosecond order, the internal electrons interact with phonons, resulting in a lattice temperature. Elevated, according to the Debye-Waller effect, the intensity changes in the dark field image and therefore the sample diffraction changes. At the same time, the increase of temperature leads to changes such as expansion and tilt of the sample lattice, which also leads to changes in the diffraction of the sample, and thus changes in the intensity in the dark field image. Therefore, the diffraction contrast (diffraction contrast) of the time-resolved dark field image of the present disclosure in the ultrafast process contains two effects, which can be used for the study of two different processes. The present disclosure has very good contrast and spatial resolution, and can directly observe different change processes inside the gold nanoparticles. Therefore, the time-resolved dark-field image obtained in the present disclosure has more obvious contrast, higher spatial resolution, and can be observed Dynamics of lattice vibrations and phonon interactions in nanoscale samples in real space.

In some embodiments, compared with the brightfield mode of ultrafast transmission electron microscopy, the present disclosure can select different diffraction spots of the sample to image, so as to obtain different crystal plane change information, as shown in FIG. 7B is the diffraction pattern of the Au nanoparticle sample, including Au For the diffraction spots of the particles and the substrate graphene, the {220} crystal plane of the Au nanoparticle is selected as the diffraction spot imaging, and the other diffraction spots can also be selected for imaging, such as {200}{400}.

In some examples, the substrate will have an effect on the ultrafast process. Depending on the substrate, the ultrafast action will behave differently. The present disclosure can study the substrate scattering signal in the ultrafast action process. At the same time, the present disclosure includes phonon interactions, which can be The sample-substrate interaction in ultrafast is indirectly reflected, so the present disclosure can study different substrate effects.

In reciprocal space, large objective apertures will contain scattered electrons at more angles, so this disclosure includes both diffraction contrast signals from crystalline samples and thermal scattering signals from amorphous regions, and can therefore also be used to study substrates Effects on ultrafast processes.

In some embodiments, the photogenerated electrons converge on the sample in an approximate convergent beam mode to obtain approximate convergent beam diffraction of the sample. The objective lens aperture is selected to cover the diffraction spot. The larger the objective lens aperture size is, the larger the angle will be in the dark field image. Depending on the size of the aperture of the objective lens, the resulting dark field image contains thermally scattered electrons at different angles, and different ranges of thermally scattered electrons can be selected for imaging, for example, the aperture size of the objective lens in the range of 20 microns to 40 microns can be used .

In some embodiments, the present disclosure can clearly show the existence of defects in the sample, and the phonon interaction process is more obvious, and the influence of defects on the interaction of electrons and phonons can be studied, such as point defects, dislocations, etc., or different The interaction between grains, or the influence of different phases, the present disclosure can directly characterize the change process of the non-uniform region inside the material.

In some examples, the present disclosure can be used to study the change process of different regions inside the metal nanoparticle, and the present disclosure can directly observe the change process of a specific region with high spatial resolution. In some embodiments, the present disclosure can also study the internal change process of materials with different constituent elements, different structures, and different morphologies. The material morphologies can be one-dimensional materials, such as nanowires, nanorods, nanoribbons, etc., or two It is suitable for the study of various materials, such as metal elements and alloys, such as gold, silver, copper, etc. and their alloys, transition metal compounds, zinc oxide, molybdenum oxide, molybdenum sulfide, tungsten selenide, etc. The present disclosure is not limited to the aforementioned substances.

Conventional TEM systems require electrons to irradiate the sample in an approximately parallel beam, and use the smallest size of the objective lens to reduce scattered electrons in the dark field image. Since the photogenerated electrons in the ultrafast mode irradiate the sample in the form of pulses, the number of electrons is much less than that of the conventional electron microscope. It is difficult to directly apply this method to ultrafast transmission electron microscopy, resulting in poor dark field image contrast and high requirements on sample size.

The method and application of obtaining a time-resolved dark-field image based on an ultrafast transmission electron microscope system proposed in the present disclosure, by adopting a high-convergence angle incident beam imaging, using a large-sized objective lens aperture (for example, 20 microns to 40 microns), and using diffracted electrons as signals For imaging, compared with the ultrafast TEM bright field image, the dark field image with opposite contrast reduces the background signal, thereby improving the signal-to-noise ratio of the acquired signal, and can more directly observe different areas inside the sample compared to the diffraction mode signal. It has high spatial resolution and expands the range of ultrafast electron microscopy to characterize samples in dark field, and can analyze the details of contrast changes after particles are excited by laser. The time-resolved dark field image obtained by the present disclosure has higher contrast and more obvious lattice changes, and can observe the dynamic change process of the sample in ultrafast time scales, such as picoseconds, nanoseconds, or microseconds. It contains the interaction information of electrons and phonons and the tilting information of the lattice direction, as well as the heat scattering electrons, which can be used for the analysis of heat scattering electron signals. The image acquisition process of the present disclosure adopts automatic acquisition, and different reference modes can be selected to ensure that the acquisition process reduces external interference and improves acquisition efficiency. The present disclosure also proposes a corresponding data processing and analysis program, which can analyze the collected data images in batches, perform drift correction on the sample position, perform noise removal and filtering processing on the image, output single pixel intensity analysis, regional pixel intensity analysis, line intensity analysis Analysis, output video, etc.

Unless specifically described or steps must occur sequentially, the order of the above steps is not limited to those listed above, and may be varied or rearranged according to the desired design. And the above embodiments can be mixed and matched with each other or with other embodiments based on the consideration of design and reliability, that is, the technical features in different embodiments can be freely combined to form more embodiments.

The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present disclosure in detail. It should be understood that the above-mentioned specific embodiments are only specific embodiments of the present disclosure, and are not intended to limit the present disclosure. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure should be included within the protection scope of the present disclosure.

Claims (10)

1. a method for obtaining a time-resolved dark field image based on an ultrafast transmission electron microscope system, is characterized in that, comprising: Step S12 : in the thermionic mode, the approximate double beam diffraction of the sample is obtained by adjusting and a dark field image is obtained by using an objective lens aperture to cover the diffraction spot; Step S13: in the photoelectron mode, the electron beam spot is converged with a larger convergence angle, and the convergent beam diffraction of the sample is obtained in the diffraction mode; in the dark field mode, based on the tilt angle of the incident electron beam in the thermionic mode, the objective aperture is used The diffraction spot is selected to obtain a time-resolved dark field image of the sample in the photoelectron mode, wherein the larger convergence angle makes the diffraction spot area in the photoelectron mode larger than the diffraction spot area in the thermionic mode. 2. The method according to claim 1, wherein step S12 comprises: Sub-step S12a: select the sample area to be characterized in the thermionic mode; Sub-step S12b: use the selective aperture to cover the sample, enter the diffraction mode, adjust the diffraction spot to the center of the screen, and adjust the tilt angle of the sample so that the diffraction pattern under the electron microscope system is double-beam diffraction; Sub-step S12c: enter the dark field mode, adjust the incident angle of the incident electron beam so that the corresponding diffraction spot is located at the position of the original transmission spot, and record the current tilt angle of the incident electron beam; Sub-step S12d: selecting the objective lens aperture to cover the diffraction spot, and exiting the diffraction mode to obtain a dark field image; Sub-step S12e: fine-tune the tilt angle of the sample according to the dark field image, so that the region of interest is a bright region; The above sub-steps S12b to S12e are performed alternately until the target area where the dark field image is obtained is in a bright state, and the diffraction spot is in a double-beam diffraction pattern. 3. The method according to claim 1 or 2, wherein the step S13 comprises: Sub-step S13a: in the optoelectronic mode, converging the photogenerated probe electron pulses on the sample, approaching the converging beam mode; Sub-step S13b: enter the diffraction mode, and obtain the approximate convergent beam diffraction of the sample region under the photogenerated detection electron pulse; Sub-step S13c: enter the dark field mode, adjust the tilt angle of the electron beam according to the inclination angle of the incident electron beam recorded in the thermionic mode, move the corresponding diffraction spot to the original transmission spot position, and adjust the spot position to make the diffraction spot the highest brightness; Sub-step S13d: select the objective lens aperture to cover the central diffraction spot, and the size of the objective lens aperture satisfies: about 90% of the electrons in the diffraction spot pass through the objective lens aperture or the imaging device shows that the size of the objective lens aperture accounts for 75% of the diffraction spot size. % or more; exit the diffraction mode to obtain a dark field image; Sub-step S13e: Adjust the position of the electron beam, adjust the size of the electron beam spot, and obtain the best dark field image. 4. The method according to any one of claims 1 to 3, wherein the objective lens diaphragm is a large-sized objective lens diaphragm, and the diaphragm aperture of the large-sized objective lens diaphragm is between 10 microns and 100 microns between; Preferably, the aperture of the objective lens diaphragm is between 20 microns and 40 microns. 5. The method according to any one of claims 1 to 4, characterized in that, before step S12, further comprising: Step S11: in the ultrafast transmission electron microscope system, adjust the focus position of the pump laser to focus on the sample position, and focus the detection laser on the filament to generate a detection electron pulse; Preferably, the step S11 includes: Before the pump laser enters the sample chamber of the TEM system, add a visible light reflector to reflect the pump laser to the beam analyzer, adjust the position of the beam analyzer to find the pump laser focus position and focus spot size, and obtain the actual focus point and focus lens. distance and focus spot size; According to the focus position, adjust the position of the focus lens in the pump optical path through the three-dimensional displacement stage; A carbon film is placed in the ultrafast transmission electron microscope, and the position of the carbon film is excited by the minimum energy pump light, so that the excitation position of the pump laser coincides with the position of the photogenerated detection electrons; and Finely adjust the focus lens position to find the best focus position. 6. The method according to any one of claims 1 to 5, wherein after the step S13, the method further comprises: Step S14: using an automatic collection module to collect images, and the automatic collection module realizes its functions through software, or hardware, or a combination of hardware and software. 7. The method according to claim 6, wherein in step S14, the time-resolved dark field image is automatically collected and saved by controlling the one-dimensional electric displacement stage and the transmission electron microscope system imaging device, and the method for automatic collection and storage is carried out. include: Sub-step S141a: setting the ultrafast transmission electron microscope system according to the parameters set by the user; Sub-step S141b: the automatic acquisition module controls the one-dimensional electric displacement stage to move to a specified position; adjusts the optical path of the pump laser, and controls the interval time between the pump laser pulse reaching the sample and the detection electron pulse reaching the sample; Sub-step S141c: the automatic acquisition module acquires the image according to the parameters, assigns an ID to the image, and saves the image to a local file; Sub-step S141d: determine whether to collect a reference image, if yes, execute sub-step S141e, otherwise execute sub-step S141g; Sub-step S141e: control the one-dimensional electric displacement stage to move from the current position P to the negative time position, the above-mentioned current position P is used as the original position of the reference, and the reference image is collected and saved; Sub-step S141f: control the one-dimensional electric displacement stage to return to the original position P, re-collect the image and save it; Sub-step S141g: the system judges whether the acquisition process is completed, and if so, ends the acquisition process, otherwise controls the one-dimensional electric displacement stage to the next position to acquire images; Sub-step S141b to sub-step S141g are repeated, and the acquisition process of all images is finally completed. 8. The method according to claim 6, wherein in this step S14, the pump laser is controlled by controlling the mechanical shutter switch in the pump laser light path, and the reference image and the excitation image are collected; comprising: Sub-step S142a: control the imaging system of the transmission electron microscope system according to the parameters set by the user; Sub-step S142b: the automatic acquisition module controls the mechanical shutter to close, and the sample is in an unexcited state; Sub-step S142c: the automatic acquisition module controls the one-dimensional displacement stage to move to a designated position; Sub-step S142d: the automatic acquisition module acquires a reference image according to the parameters, assigns an ID to the image, and saves the image in a local folder; Sub-step S142e: the automatic acquisition module controls the opening of the mechanical shutter, the pump laser is introduced into the sample area of the TEM system, and the sample is excited by the laser to produce an instantaneous change; Step S142f: the automatic acquisition module acquires the sample excitation image according to the parameters, assigns an ID to the image and saves it to another local folder; Sub-step S142g: the system judges whether the acquisition process is completed, and if so, ends the acquisition process, otherwise controls the one-dimensional electric displacement stage to the next position to acquire images; Sub-steps S142b to S142g are repeated, and the sample state when the sample is excited by the pump laser pulse and the sample state when it is not excited are taken as reference signals at different intervals to complete the acquisition process of all images. 9. The method according to any one of claims 6 to 8, wherein the step S14 further comprises: Batch processing of acquired images; Wherein, the batch processing of the collected images includes one or more of image averaging as a reference image, image offset calibration, image intensity analysis and result output; The image intensity analysis includes: one or more of single pixel intensity analysis, area intensity analysis, and line intensity analysis; The result output includes one or more of image noise processing output, intensity curve output, position calibration output, image cross-correlation value output, image drift amount output, Fourier analysis output and video output. 10. A time-resolved dark-field image obtained by the method of any one of claims 1 to 9 is used to analyze the details of the contrast change of nanoparticles after excitation by laser light, and to characterize crystals of different materials in an ultrafast time scale. Application of lattice-oriented lattice relaxation and resonance dynamics processes.

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