Method for adjusting haadf image of spherical aberration transmission electron microscope for quickly positioning sample thin area

Abstract

This invention discloses a method for rapidly locating thin regions of samples using a spherical aberration transmission electron microscope (HAADF) and adjusting the image. The steps are as follows: 1) In HAADF imaging mode, move the sample stage at low magnification to find unexposed samples and perform high-magnification atomic-resolution imaging on them. If the image signal-to-noise ratio is low and details are blurred due to excessive sample thickness, increase the image brightness or contrast until the sample signal is overexposed. 2) Reduce the magnification and continue searching for unexposed samples. Perform high-magnification atomic-resolution imaging on them. If the image signal-to-noise ratio is still low due to excessive sample thickness, continue to increase the brightness or contrast to overexpose the sample. 3) Repeat step 2) to find new unexposed locations, gradually filtering out thicker samples. Finally, a sample that is found and magnified to obtain an atomic-resolution image with high signal-to-noise ratio and clear details is identified as the target thin region of the sample. This adjustment method is efficient and accurate.

Description

Technical Field

[0001] This invention belongs to the field of transmission electron microscopy, specifically relating to a method for adjusting HAADF images to rapidly locate thin regions of samples using spherical aberration transmission electron microscopy. Background Technology

[0002] In transmission electron microscopy (TEM) characterization, to obtain high-quality images, especially atomically resolved or atomically scaled images, the sample must be sufficiently thin, typically less than 100 nm, to ensure effective electron beam penetration and reduce multiple scattering. Smaller sample thickness generally results in better image contrast, resolution, and signal-to-noise ratio. However, most TEM samples are not uniformly thick, requiring the identification of thin regions for imaging. For example, many single-atom catalyst samples are often sheet-like or bulky with highly non-uniform thickness. Thin regions suitable for imaging often exist only at the edges, wrinkles, or fractures of the sample, and are small and randomly distributed. Obtaining atomically resolved HAADF images requires imaging at relatively thin locations within the sample; the thinner the sample, the higher the image quality.

[0003] In conventional high-angle annular dark-field (HAADF) imaging mode, the intensity of the HAADF image for samples with homogeneous chemical composition is positively correlated with the sample thickness. When searching for samples at low magnification with a large field of view, the field of view contains a large number of samples. Generally, to ensure that all signals from the thickest part of the sample to the blank support film are within the dynamic range, the operator will adjust the brightness and contrast of the image to a "safe" range (the Auto function of the imaging software). This leads to a critical problem: when the sample thickness varies greatly, the signal intensity generated at the thinnest part of the sample and the signal intensity of the pores in the support film will be compressed to very close gray values ​​when displayed at low magnification, making the thin area almost indistinguishable from the background by the human eye. Therefore, when searching for samples at low magnification with a large field of view, the operator has to rely on experience, repeatedly and slowly moving and searching for the thinnest area in the low-contrast image. This process is time-consuming, laborious, and has a low success rate, severely limiting the characterization efficiency, especially in high-throughput screening or rapid observation of electron beam sensitive samples.

[0004] Currently, there is no method for quickly locating thin regions in samples with uneven thickness. The common approach is to find a relatively dark sample location at low magnification, gradually increasing the magnification and continuing to locate the darkest sample location within the field of view. However, due to the randomness of sample dispersion, it cannot be guaranteed that the darkest sample location within a small field of view is the thin region. If it is not a thin region, a high-quality image cannot be obtained, and the magnification must be reduced again to search for other dark areas until the thin region is found. This method cannot quickly locate thin regions within a large field of view at low magnification; it requires magnifying each position of the sample on the mounting screen to search for the thin region. Whether a thin region is found and how long it takes to find it are entirely random. Furthermore, spherical aberration transmission electron microscopy characterization usually requires finding multiple thin region locations to obtain atomically resolved images to avoid selective region imaging. The above methods are highly random, and finding multiple thin region locations is very time-consuming. Summary of the Invention

[0005] To address the problems of low efficiency and reliance on experience in locating thin sample regions in existing technologies, this invention provides a HAADF image adjustment method for rapid localization of thin sample regions using spherical aberration transmission electron microscopy. This method utilizes the positive correlation between HAADF image signal intensity and thickness, and increases the brightness / contrast during imaging in a stepwise manner. Excessively thick samples are marked as "overexposed" in the image and actively excluded, thereby systematically guiding the operator to quickly locate thinner sample positions that are more suitable for atomic resolution imaging.

[0006] This invention provides a method for rapidly locating thin regions of a sample using HAADF image adjustment in spherical aberration transmission electron microscopy, comprising the following steps: 1) In the HAADF imaging mode of the condenser lens spherical aberration corrected transmission electron microscope, move the sample stage within a low magnification and large field of view to find the sample. Gradually magnify the imaging magnification to perform atomic-level resolution imaging of the sample. If it is determined that the low signal-to-noise ratio and blurred details are caused by the sample being too thick at that position, increase the brightness and contrast of the HAADF image until the sample signal at that position appears as a uniform, detail-free bright white on the monitor, which is the "overexposed" state. 2) Reduce the magnification to a low magnification with a large field of view and move the sample stage to continue searching for samples with unexposed signals in the real-time image; perform high-magnification atomic resolution imaging on the unexposed samples. If it is determined that the current image signal-to-noise ratio is low and the details are blurred due to the sample being too thick, then continue to increase the brightness and contrast so that the sample at that position also enters the "overexposed" state. 3) Repeat step 2) by cyclically filtering out thicker areas by increasing the brightness contrast to overexpose the current thick sample and moving the sample stage to find new samples that are not overexposed. Finally, when the sample is moved to a position that is not overexposed, magnify the image to obtain an atomically resolved image with a high signal-to-noise ratio, a clean background, and clear details. This position is the target thin area. Maintain the current brightness contrast and continue to search for multiple samples that are not overexposed to avoid selective region imaging.

[0007] As a further improvement of the present invention: a sample with non-uniform thickness refers to a material whose physical thickness is non-uniform at the nanoscale, but whose chemical composition is uniform.

[0008] As a further improvement of the present invention: before starting step (1), optical path alignment, spherical aberration optimization and sample decarbonization should be completed to ensure that the electron microscope is in the best working condition. The specific steps are as follows: (1) Optical path alignment, spherical aberration optimization, and sample decarbonization: First, optical path alignment and spherical aberration parameter optimization were performed on the electron microscope using a gold standard sample. Then, the sample-loaded grid was mounted on the sample holder of the transmission electron microscope, inserted into the microscope, and the STEM mode was selected. The sample was decarbonized using the beam shower method. After decarbonization, A1 / B2 was adjusted according to the Ronchigram diagram at the amorphous carbon area to increase the smooth area. The smooth area was covered with a condenser aperture, and the HAADF probe was inserted.

[0009] As a further improvement of the present invention: the statement in step (1) that "if it is determined that the image signal-to-noise ratio is low and the details are blurred due to the excessive thickness of the sample at this position" means that the image details are blurred not because of sample shaking or carbon contamination. The edges of a jittery sample are blurred in atomically resolved images; Image detail blurring caused by carbon pollution can be addressed by continuous high-magnification electron beam scanning. If the image becomes increasingly bright and blurry, it indicates poor image quality due to carbon pollution. The image with a thick sample has a low signal-to-noise ratio and blurry details. The sample edges are clear, but the details inside the sample are blurry, and the image does not brighten continuously as the electron beam continues to scan.

[0010] As a further improvement of the present invention: the "unexposed sample" mentioned in steps 2) and 3) is a sample whose image grayscale changes under the current display settings, and whose details can be made clear by adjusting the objective lens focus.

[0011] As a further improvement of the present invention: in step 3), the “thin area” refers to a sample location that is thin enough to meet the requirements of atomic resolution imaging.

[0012] As a further improvement of the present invention: the sample includes amorphous sample and crystalline sample.

[0013] It should be noted that for amorphous samples with uniform chemical composition, the HAADF signal intensity is strictly positively correlated with the sample thickness, making them the most ideal and direct application targets for this method. For crystalline samples, the HAADF image intensity is simultaneously affected by both mass-thickness contrast and diffraction contrast. Under specific crystal orientations, diffraction effects may modulate the HAADF intensity to some extent, causing nonlinear fluctuations in the intensity-thickness relationship for samples of moderate thickness. However, the ultimate goal of this method is to find sufficiently thin sample regions (close to or smaller than the inelastic mean free path of electrons) for atomically resolved HAADF imaging. When the sample thickness is reduced to this scale, electron multiple scattering weakens, the influence of diffraction effects is significantly reduced, and mass-thickness contrast becomes the dominant mechanism again. Therefore, the core strategy of this method, 'gradually increasing brightness contrast to eliminate thick regions,' is also efficient and effective in finding suitable thin regions for crystalline samples. Furthermore, the technical terminology in this invention refers to JY / T0581-2020 "General Rules for Analytical Methods of Transmission Electron Microscopy".

[0014] Table 1 shows a comparison between this method and conventional methods: Table 1 Comparison between this method and conventional operating methods

[0015] The beneficial technical effects of this invention are reflected in the following aspects: 1. High efficiency and intuitiveness: It transforms the abstract thickness judgment into an intuitive visual criterion of "overexposure" and "underexposure" of the image, which greatly reduces the operation threshold, reduces the reliance on personal experience, and can systematically and quickly approximate thin areas, significantly shortening the search time.

[0016] 2. High versatility: The principle of this method is based on the basic physical principle of HAADF signal. It is applicable to all transmission electron microscopes equipped with condenser lens spherical aberration correctors and HAADF detectors. It is especially suitable for spherical aberration electron microscopy characterization of various non-noble metal single-atom catalysts that need to be imaged with sufficiently thin samples. Its value in improving efficiency is even more prominent.

[0017] 3. Sample protection: Rapid positioning reduces the ineffective dwell time of the electron beam at non-target locations, thereby reducing the potential radiation damage risk to electron beam sensitive samples (such as MOFs and certain catalysts).

[0018] 4. Cost savings: Once the thin region is located, all unexposed sample locations are considered thin regions, which facilitates rapid multi-position imaging, improves the efficiency of high-end electron microscope equipment, and saves valuable machine time costs. Attached Figure Description

[0019] Figure 1This is a flowchart illustrating the operation of the present invention.

[0020] Figure 2 High and low magnification HAADF images of Co single-atom catalyst samples on amorphous nitrogen-doped carbon support: (a) low magnification HAADF image of the sample, (b) high magnification HAADF image of the sample obtained from the unexposed sample position found in low magnification image (a), (c) low magnification HAADF image of the sample after increasing brightness and contrast, (d) high magnification HAADF image of the sample obtained from the unexposed sample position found in low magnification image (c) (HAADF image of the thin sample area).

[0021] Figure 3 High and low magnification HAADF images of crystalline lead titanate samples: (a) Low magnification HAADF image of the sample; (b) High magnification HAADF image of the sample obtained from the unexposed sample location found in low magnification image (a); (c) Low magnification HAADF image of the sample after adjusting brightness and contrast; (d) High magnification HAADF image of the sample obtained from the unexposed sample location found in low magnification image (c); (e) Low magnification HAADF image of the sample after adjusting brightness and contrast; (f) High magnification HAADF image of the sample obtained from the unexposed sample location found in low magnification image (c) (HAADF image of the thin sample area). Red arrows indicate switching from low magnification sampling to high magnification imaging, and green arrows indicate switching from high magnification imaging to low magnification sampling. Detailed Implementation

[0022] The following description, in conjunction with the accompanying drawings, provides a further explanation of the HAADF image adjustment method for rapid localization of thin regions of samples using spherical aberration transmission electron microscopy, based on embodiments.

[0023] The equipment used in the following examples is described below: The transmission electron microscope is a Thermo Fisher Scientific Themis Z spherical aberration corrected transmission electron microscope.

[0024] Example 1 The HAADF image adjustment method for rapid localization of thin regions of samples using aberration-induced transmission electron microscopy in Example 1 is applied to HAADF imaging of Co single-atom catalyst samples on amorphous nitrogen-doped carbon supports. The specific operation steps are as follows: (2) Optical path alignment, spherical aberration optimization, and sample decarbonization: First, optical path alignment and spherical aberration parameter optimization were performed on the electron microscope using a gold standard sample. Then, the sample-loaded grid was mounted on the transmission electron microscope sample holder, inserted into the microscope, and the STEM mode was selected. The sample was decarbonized using the beam shower method. After decarbonization, A1 / B2 was adjusted according to the Ronchigram diagram at the amorphous carbon area to increase the smooth area. The smooth area was covered with a condenser aperture, and the HAADF probe was inserted.

[0025] (3) Locating a thin sample area: Click "Scan," and the electron beam will begin scanning and imaging. Rotate the magnification control button to reduce the imaging magnification (within the range of 5000 to 20000x). Locate the sample under low magnification and a large field of view, as shown in Figure 2(a). Find a location on the sample that is not overexposed, as shown in the red box in Figure 2(a). Adjust the magnification to a high level (greater than 2,500,000x, to achieve atomic-level resolution). Adjust the focusing mirror astigmatism to obtain an atomic-level HAADF image of the sample at that location, as shown in Figure 2(a). Figure 2 As shown in Figure (b), the image shows a strong signal against the carrier background, but weak single-atom signals. This indicates that the sample is too thick at this location, affecting single-atom imaging. Subsequently, the brightness and contrast of the HAADF probe were increased to overexpose the sample signal at this location.

[0026] (3) Continue searching for thin-area samples: Reduce the magnification and continue searching for new samples where the signal is not overexposed, as shown in the red box in Figure 2(c). Perform high-magnification imaging on this sample to obtain a high-magnification atomic-level HAADF image of the sample at this location. Figure 2 As shown in Figure (d), the image shows a weak background signal on the carrier and a strong single-atom signal, indicating high image quality and a high signal-to-noise ratio. Therefore, this location is considered to be a thin region of the sample. If high-magnification HAADF images of multiple thin regions are needed, the sample stage can be moved to find multiple unexposed sample locations to obtain high-magnification HAADF images of the sample.

[0027] Example 2 The HAADF image adjustment method for rapid localization of thin regions of samples using spherical aberration transmission electron microscopy in Example 2 is for HAADF imaging of crystalline lead titanate samples. The specific operation steps are as follows: (1) The adjustment of the light path and the carbon removal steps are completely consistent with those in Example 1.

[0028] (2) Locating thin-area samples: Click scan, the electron beam begins scanning and imaging. Rotate the magnification control button to reduce the imaging magnification to 5000 to 20000 times. Locate samples under low magnification and a large field of view. Figure 3 As shown in Figure (a), locate a specific unexposed sample position as shown in Figure (a). Figure 3 As shown in the red box in (a), rotate the band axis to the correct position and adjust the magnification to obtain an atomic-level HAADF image of the sample. Figure 3 As shown in Figure (b), the image shows a strong signal against the background of the carrier, but the signals of small titanium atoms are unclear. This indicates that the sample is too thick at this location, making it impossible to obtain an image with a high signal-to-background ratio. Subsequently, the brightness and contrast of the HAADF probe were increased to overexpose the sample signal at this location.

[0029] (4) Continue searching for thin-area samples: reduce the magnification and look for new locations where the signal is not overexposed, such as... Figure 3As shown in the red box in Figure (c), the sample at this location was rotated to the correct band axis, and a high-magnification atomic-level HAADF image of the sample was obtained. Figure 3 As shown in Figure (d), the image shows a strong background signal from the carrier, but the titanium atom signal remains unclear. This indicates that the sample at this location is still too thick, making it impossible to obtain an image with a high signal-to-background ratio. Subsequently, the brightness and contrast of the HAADF probe were increased to overexpose the sample signal at this location.

[0030] (5) Continue searching for thin-area samples: reduce the magnification and look for samples where the signal is not overexposed, such as... Figure 3 As shown in the red box in Figure (e), the sample at this location was rotated to the correct axis, and a high-magnification HAADF image of the sample was obtained through high-magnification imaging. Figure 3 As shown in Figure (f), the image shows a weak background signal and a strong atomic column signal, indicating high image quality and a high signal-to-noise ratio. Therefore, this location is considered a thin region of the sample. If high-magnification HAADF images of multiple thin regions are needed, the sample stage is moved to find multiple unexposed samples to obtain their high-magnification HAADF images. Note: In this invention, brackets are used to further explain the defined terms.

[0031] It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the invention. Furthermore, it should be understood that after reading the technical description of this invention, those skilled in the art can make various modifications, alterations, and / or variations to the invention, and all such equivalent forms also fall within the scope of protection defined by the appended claims. As is known from common technical knowledge, the invention can be implemented through other embodiments that do not depart from its spirit or essential characteristics. Therefore, the embodiments disclosed above are merely illustrative in all respects and are not exhaustive. All changes within the scope of this invention or equivalent to the scope of this invention are included in this invention.

Claims

1. A method for rapidly locating thin regions of samples using spherical aberration transmission electron microscopy (TEM), characterized in that... Includes the following steps: 1) In the HAADF imaging mode of the condenser lens spherical aberration corrected transmission electron microscope, move the sample stage within a low magnification and large field of view to find the sample. Once a sample that is not overexposed is found, gradually increase the imaging magnification to perform atomic-level resolution imaging of the sample. If it is determined that the low signal-to-noise ratio and blurred details are caused by the sample being too thick at that location, increase the brightness and contrast of the HAADF image until the sample signal at that location appears as a uniform, detail-free bright white on the monitor, which is the "overexposed" state. 2) Reduce the magnification to a low magnification with a large field of view and move the sample stage to continue searching for samples with unexposed signals in the real-time image; perform high-magnification atomic resolution imaging on the unexposed samples. If it is determined that the current image signal-to-noise ratio is low and the details are blurred due to the sample being too thick, then continue to increase the brightness and contrast so that the sample at that position also enters the "overexposed" state. 3) Repeat step 2) by cyclically "increasing brightness and contrast to overexpose the current thick sample, and moving the sample stage to find new samples that are not overexposed" to gradually filter out thicker areas; finally, when moving to a certain unexposed sample position, magnify the image to obtain an atomically resolved image with high signal-to-noise ratio, clean background, and clear details. This position is the target thin area; maintain the current brightness and contrast, continue to search for multiple unexposed samples to avoid selective region imaging, and obtain atomically resolved images of multiple thin areas of the sample with high signal-to-noise ratio, clean background, and clear details.

2. The HAADF image adjustment method for rapid localization of thin regions of samples using spherical aberration transmission electron microscopy according to claim 1, characterized in that: The term "non-uniform thickness sample" refers to a material whose physical thickness is non-uniform at the nanoscale, but whose chemical composition is uniform.

3. The HAADF image adjustment method for rapid localization of thin regions of samples using spherical aberration transmission electron microscopy according to claim 1, characterized in that: Step 1) Before starting, optical path alignment, spherical aberration optimization, and sample decarbonization should be completed to ensure the electron microscope is in optimal working condition. The specific steps are as follows: Optical path alignment, spherical aberration optimization, and sample decarbonization: First, optical path alignment and spherical aberration parameter optimization were performed on the electron microscope using a gold standard sample. Then, the sample-loaded grid was mounted on the sample holder of the transmission electron microscope, inserted into the microscope, and the STEM mode was selected. The sample was decarbonized using the beam shower method. After decarbonization, A1 / B2 was adjusted according to the Ronchigram diagram at the amorphous carbon area to enlarge the smooth area. The smooth area was then covered with a condenser aperture, and the HAADF probe was inserted.

4. The HAADF image adjustment method for rapid localization of thin regions of samples using spherical aberration transmission electron microscopy according to claim 1, characterized in that: The statement in step 1) that "if the image signal-to-noise ratio is low and the details are blurred due to the sample being too thick at that location" refers to images where the blurring is not caused by sample jitter or carbon contamination. The method for distinguishing between these conditions is as follows: The edges of a jittery sample are blurred in atomically resolved images; Image detail blurring caused by carbon pollution can be addressed by continuous high-magnification electron beam scanning. If the image becomes increasingly bright and blurry, it indicates poor image quality due to carbon pollution. The image with a thick sample has a low signal-to-noise ratio and blurry details. The sample edges are clear, but the details inside the sample are blurry, and the image does not brighten continuously as the electron beam continues to scan.

5. The HAADF image adjustment method for rapid localization of thin regions of samples using spherical aberration transmission electron microscopy according to claim 1, characterized in that: The "unexposed sample" mentioned in steps 2) and 3) refers to a sample whose image grayscale changes under the current display settings, and whose details can be made clear by adjusting the objective lens focus.

6. The HAADF image adjustment method for rapid localization of thin regions of samples using spherical aberration transmission electron microscopy according to claim 1, characterized in that: In step 3), the "thin region" refers to a sample location that is thin enough to meet the requirements of atomic resolution imaging.

7. The HAADF image adjustment method for rapid localization of thin regions of samples using spherical aberration transmission electron microscopy according to claim 1, 2, 4, or 6, characterized in that: The samples include amorphous samples and crystalline samples.

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