Research Nuggets - FAQs about TEM

1.Differences in Imaging Between TEM and SEM

TEM imaging uses transmitted electrons. Accelerated electrons can pass through thin film samples and reach a detection screen. The contrast in the image is generated by mass thickness (different elements, element content, and thickness of the sample) and diffraction thickness (different crystal orientations). SEM imaging, on the other hand, uses reflected electrons (secondary electrons). Accelerated electrons are reflected by the sample surface and reach the detection screen. Due to the unevenness of the sample surface, i.e., different tilt angles, the density of electrons reaching the detection screen varies, thus creating image contrast. In backscatter imaging, differences in element type and content lead to different electron scattering, which also affects image contrast. Additionally, Auger electron imaging can also reflect differences in element type and content, further influencing image contrast.

2.Difference between TEM and XRD: Why can PDF cards be used in both types of microscopes? Are the dynamics of X-rays and electrons the same? Will it affect the peaks? Are the spacings of the diffraction rings the same for both? What are the differences?

In TEM/SEM, when the electron beam bombards the material, different elements produce characteristic X-rays with different energies. The energy spectrum probe in TEM/SEM can detect the energy of X-rays (using a signal amplifier). Based on the energy of the X-rays, it is possible to determine which element they originated from, and the intensity of the X-rays can be used to determine the element's content. In contrast, XRD uses a copper or cobalt target to generate strong characteristic X-rays with consistent intensity and wavelength (the element producing these X-rays is already known, and it is specifically selected because of the strong intensity and minimal wavelength variation). By examining the crystal structure and orientation of the detection material, the diffraction of X-rays differs, similar to how a mirror reflects light differently depending on its tilt angle. In XRD, the detector angle is fixed, and the angle of incident X-rays ranges from 0° to 145° in steps or continuously varies. Different crystal planes of the material may produce diffraction at different positions. When the incident X-rays are just right and enter the detector at specific diffraction angles, the diffraction intensity variation curve appears in the XRD measurement results, while TEM and SEM provide information about the element content ratio.

3.If you want to make carbon quantum dots with a size below 10nm using ultra-thin carbon films, what should you pay attention to when preparing the samples?

Dissolve the sample in a specific solution, then drop it onto the ultra-thin carbon film, let it air dry naturally, and then you can observe it in the TEM. When preparing the sample, pay attention to the appropriate concentration of the sample in the solution, and stir it appropriately to ensure uniform distribution in the solution. After dropping it onto the ultra-thin carbon film, ensure that the distribution is uniform and of appropriate density to prevent overlapping or too sparse samples. After dropping the sample, let it air dry naturally as much as possible to prevent damage to the ultra-thin carbon film. When handling the sample, try to clip it at the edge to prevent damage. When observing it in the TEM, pay attention to lower light intensity and shorter observation times to prevent the electron beam from damaging the sample due to excessive light intensity.

4.When preparing thin film samples, what is used to stick the sample to a very flat block during pre-thinning and final thinning, and how is it separated? Also, how is the thickness of the sample measured after grinding to a certain thickness?

The sample can be glued to a very flat martensitic stainless steel block (e.g., a steel block with a thickness of 6 millimeters and a diameter of 6 centimeters) using 502 glue. Martensitic steel is chosen because of its high hardness, which reduces wear during sample preparation. The steel block and the sample can be separated by placing them in acetone and sonicating for 5 to 10 minutes. The thickness of the sample after pre-thinning can be measured using a micrometer. Gradually thin the sample from 0.5 millimeters to 50 micrometers using sandpaper ranging from 400 to 5000 grit. The final thickness after thinning is often difficult or unnecessary to measure. As long as clear TEM images can be observed, the sample preparation is considered good. A small trick is to use an optical microscope to observe the perforated sample after pre-thinning or ion thinning. By focusing on the edge of the perforation, if there is a very smooth transition, it often indicates that the TEM sample is of good quality. If there is significant deformation or a very flat cut, it often indicates poor sample preparation.

5.Methods for TEM Data Analysis

Digital Micrograph is the most commonly used TEM analysis software. The general steps for calibration of diffraction spots using DM software are:

1）Determine the lattice constants and interplanar spacing of all crystal planes. 2）Measure the spacing between multiple diffraction spots and calculate the interplanar spacing, then compare it with the theoretical value. 3）Measure the angles between different diffraction spots corresponding to crystal planes and compare them with the theoretical value. Common data meanings in DM software:

In the control panel, "L" refers to the reciprocal of the interplanar spacing, so 1/L is the numerical value of the interplanar spacing. "R" refers to the angle between the diffraction spot corresponding to the straight line you draw in the image and the horizontal direction. The sum or difference of the angles between the two straight lines can determine the angle between different diffraction spots corresponding to crystal planes.