ISO/TS 22292:2021 Nanotechnologies — 3D image reconstruction of rod- supported nano-objects using transmission electron microscopy.
5.1.2 Acceleration voltage
The acceleration voltage shall be selected as described in A.2.4 d) on sample thickness. Typically, 300 kV or 200 kV should be used.
Using the maximum voltage available on a particular TEM is preferred. Using the maximum available voltage means maximum allowable sample rod diameter and maximum depth of focus. For example, using 300 kV rather than 200 kV allows for 250 nm diameter carbon rod rather than 200 nm diameter rod at 200 kV.
5.1.3 Convergence semi-angle
The convergence semi-angle shall be selected so that the illumination at the sample plane is uniform over the imaged area of the sample. Furthermore, the illumination uniformity has to be such that the apparent sample focus does not visibly change over the observed area.
NOTE The illumination uniformity can be verified by performing an intensity profile across the image (e.g. diagonally from corner to corner of the image). The uniformity of the defocus value is typically not a concern and It can be verified by performing a fast Fourier transform (FFT) of sub areas of the image. For example, a 2048 x 2048 image can be divided into 256 x 256 pixels regions of interest. The image composed of absolute value of FFT can be compared among the various regions of interest to ensure that the FFT does not vary too much from region to region. An FFT that has same numbers of circular rings over an arbitrary sub-field of view is an indication that illumination is sufficiently uniform.
5.1.4 Collection angle
In conventional TEM mode the collection angle is selected by the objective aperture size. The collection angle is determined as the square root of the sum of squares of the convergence angle and acceptance angle of the objective aperture. In practise the convergence angle in TEM mode is much smaller than the acceptance angle of the objective aperture. Therefore, the collection angle is determined by the objective aperture acceptance angle alone.
The main criteria for collection angle is the avoidance of diffraction contrast while maximizing the contrast in the image. The contrast increases with decreasing collection angle,I2lI1J but at the same time, the diffraction contrast increases with decreasing collection angle. The presence of diffraction contrast could make the data unsuitable for 3D reconstruction.l9JI1J The number of counts collected by the detector decreases with decreasing collection angle leading to increase in acquisition time.
5.1.5 Microscope magnification
Microscope magnification and detector pixel size are critical parameters to ensure correct sampling of the object for 3D reconstruction. The higher the desired number sampling, the lower the effect of the detector point spread function. At the same time high sampling, i.e. high microscope magnification and small pixel size, decreases the field of view that can he acquired without region stitching by subsequent offset acquisitions. High magnification also decreases the number of counts per pixel at a given beam current density and acquisition time. Typically, the magnification is chosen so that the desired projected image resolution is sampled by 5 pixels or moreL11l.ISO/TS 22292 pdf download.