Behind the structure on our image, consisting of tiny hollow spheres, is a diatom only a few micrometers in size. It becomes visible with the help of a very special microscope.
Most diatoms are microscopic, around 50 micrometers in size. Typical for them is a solid shell made of silica, which can be box-shaped, round, thread-shaped or elliptical. This shell gets its stability from silicon dioxide crystals which, with the help of a peptide, form small spherical clusters. These nanospheres give the diatom shells their “grainy” appearance under the light microscope.
However, light microscopes are not sufficient to illuminate the exact structure of the diatom shells and their nanospheres. And the fine structure of these shells is not visible in detail even in the scanning electron microscope, because samples have to be coated with gold or other precious metals. This sticks together the tiny structural details.
On the other hand, helium ion microscopy offers a more suitable alternative. This shell of a spherical diatom was also taken up with it. For this microscopy technique
helium atoms are ionized at the tip of an ultra-fine tungsten needle and accelerated away from the needle. The helium ion beam is then bundled and directed onto the material to be examined.
In order to generate an image, the intensities of the beam passing through the sample and of the reflected beam, as well as the number of secondary electrons generated, are measured. This enables scientists to visualize the surfaces of samples only a few nanometers in size – such as the diatom on our image – in detail and with sharp focus.
“Because of these properties, focused ion beams have great potential for many other applications in nanotechnology,” explains Gregor Hlawacek from the Helmholtz Center Dresden-Rossendorf. “For example, surfaces can be flexibly structured in the nano range or local material properties can be changed in a targeted manner.”