seto77/ReciPro

query about STEM simulator Detector angles

Opened this issue · 2 comments

sandhok commented

Dear Seto,

I have a query related to STEM simulator. Is there any rule on minimum inner detector radius with respect to the convergence angle? I ask this because I am noticing that if choose inner detector radius much higher than twice of convergence angle the simulated image result is giving me just a blank dark image. For example, if my convergence angle is 21 mrad and inner and outer detector collection angles (annular detector range radius) are 79 mrad and 200 mrad the simulated result is just blank image. If I remeber correctly I didnt face this issue few months ago when I used the STEM simulator. Looking forward to hear from you. Thank you...Sandeep

seto77 commented

Thanks for asking the question.

Is there any rule on minimum inner detector radius with respect to the convergence angle?

No, there are no specific rules.

If the elastic STEM image is completely dark, the number of diffraction waves considered in the calculation may not be sufficient.
image
Especially for crystals with low symmetry, the diffraction waves are limited to low-angle regions. If the number of diffraction waves is increased, so that diffraction waves at high angles are also included in the calculation, a proper image will be displayed. (However, it is recommended that the number of diffraction waves be limited to about 200 at most, since the computational cost explodes as the number of diffraction waves increases.)

If the TDS-STEM is also completely dark, check the value of the Debye-Waller factor.
image
If the Debye-Waller factors for all atoms are zero, then there is no TDS contribution and the TDS-STEM is always dark, regardless of the detector angle value.

Usually, above 79mrad, the TDS contribution will be dominant over elastic scattering. If you do not know the exact value of the DW factor, set B=0.5 for all atoms for now and recalculate the TDS-STEM. I hope it will work.

sandhok commented

Thank you for your response. The information and suggestions you mentioned solved the issue.
Very grateful for your kind reply!