What is ronchigram in STEM and why it is important?

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Matthew Richard Field Popular answer

A ronchigram is essentially a CBED pattern of your sample.

In terms of STEM usage, the most important part of it is the centre part which is 'flat' looking. External warping of the ronchigram is due to aberrations in your microscope. (Which can be reduced using aberration correctors)

During STEM, the beam is rastered across your sample, and the intensity of the CBED pattern (at a particular angle) is taken and an image is formed from this.

A smaller condenser aperture can be used to select the part of the ronchigram which has no aberrations, making your image look better.

However, smaller apertures also lead to diffraction limited imaging, meaning you will lose some resolution that way.

Matthew Richard Field

Jyoti Gavali

When we put the electron microscope in the STEM mode,with the scan switched off,and put the condenser lens,aperture and stigmator,objective rotation centre in control.Now examine the phosphor screen at the bottom end of the microscope.The rays coming out of the sample are conical in shape formed by the condenser aperture.this cone hits the phosphor screen which forms a bright image called as ronchigram.You may find detailed answers for the size of apertures on the following link.

http://www.rodenburg.org/stem/t300.html

Aladin Ullrich

As Jyoti wrote, you may find a quite nice description of TEM/STEM and the Ronchigram on the site of Rodenburg: http://www.rodenburg.org/STEM/t200.html. I started with this page several years ago and found it quite useful.

Basically, I use the Ronchigram for the alignment when doing STEM.

Yoones Kabiri

Hi Matthew,

Regarding point number 5): smaller condenser aperture in STEM will not lead to information lost of high spatial frequency. That is true for wide-field illumination. Resolution of STEM is dependent only on probe size, which actually smaller condenser aperture and higher spot size help you to get smaller probe. But you lose a lot of probe intensity by using smaller condenser aperture. Also the convergence angle of probe gets smaller by using smaller condenser aperture. I can't remember how convergence angle is related to resolution in STEM but I'm sure you will find the relation in TEM books.