Difference in model with simulated results

[Tara Mishra]

Dear Prof. Koch,
For a theoretical study I have been simulating a hypothetical crystal. I am getting some strange results which doesn't match with the model which is being scanned. Could you please identify the problem with my parameters?
Thank you
Regards
Tara                                                                                                                                                                                                                                                                                                                                                                                        

[Manoj]

Dear Tara Mishra,

Hello. 

A STEM image is acquired by scanning a converged probe on the object. In order to image atomic columns (similar to the crystal you have used), the probe has to be positioned close to or at the centre of the atomic column. In your crystal, there are 13 atomic columns. A simulated STEM image with the current set of probe parameters decreases the intensity of some of the atomic columns. Now consider the grid of probes that you have used, it is 10x10. The probe grid starts from the top left corner and each cross-hair position represents a probe position. Notice that there are actually 121 probe positions (11x11) although 10x10 probe grid was specified. The right-most column and bottom-most row are there just to complete the scanning area and are not visited during STEM image formation. 

Due to the smaller grid sampling (10x10), some of the atomic columns do not get the probe to be close to the centre of those columns. The intensity of such columns would be diminished depending on how close a probe is to the centre of the column. Please go through the attached tutorial by M. Sarahan (available on Prof. Koch's site: https://www.physics.hu-berlin.de/en/sem/software/qstem-documentation) for guidelines on how to assign the scanning window parameters. Increasing the grid sampling such that each atomic column is visited at least once should solve the current issue, say, 20x20.

Regards,

Manoj Settem

[Mingquan Xu]

Hi, Tara,

    In your "microscope parameter", the C3 is too large. You can set a smaller C3 value,such as 0.002 mm, then click the "scherzer" icon. In addition, increase the pixels in "scanning window", such as 100x100. For a time-saving test, not choose the TDS and try.

Best regards

在 2018年2月7日星期三 UTC+8下午6:27:40，Tara Mishra写道：