They are a nanoscale size crystal grains with different directions planes

for More information you can see publications including HRTEM images particularly for Si nanoparticles

They are a nanoscale size crystal grains with different directions planes

for More information you can see publications including HRTEM images particularly for Si nanoparticles

It may be from poor vibration isolation. You could determine if it is an artifact by changing the scan speed or magnification to see if the ripples are maintained.

They look like Moire fringes. You can confirm this by tilting your specimen. If the fringes change significantly or disappear, then they are Moire fringes.

I really doubt that this are "nanoscale size crystals", Omar M S! Scalebar is 50 nm, and in such crystal planes should be then separated for approx 5 nm!

Christopher Goodwin the image was recorded in C-TEM mode, changing the scan speed would be only possible in scanning mode (STEM).

For Moire fringes, I would not expect circular patterns. But since we don't know what we are looking at (besides it should be crystalline) my wild guess would be some sort of contamination. Is it possible, that some cellulose fibers are mixed with your sample? Like from filtering paper, or from sample preparation?

Janez Zavašnik No, it does not have any contamination. This is the TEM image of a COF.