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The basis: Carbon nanomembranes

The foundation for our technology are Carbon-based molecular thin nanomembranes. A Carbon nanomembrane (CNM) is comparable to a household cling film, however it is approximately 10,000 times thinner. Like a cling film over a salad bowl, a CNM can be spanned over openings. The figure shows a free-standing CNM which was transferred to a grid with hexagonal openings of approximately 50 microns (i.e. about 10,000 time smaller than the opening of the salad bowl). The dimensions of the free-standing CNM are not only laterally smaller, but also in thickness. Apart from that, both films show similar behaviour. They are spanned under a slight tension and exhibit - when not perfectly placed - even a similar drape.

CNMs are manufactured by radiation-induced cross-linking a self-assembled monolayer of molecules on a suitable substrate. The thickness of the membrane, its perforation and a variety of other specifications can be controlled in a wide range by the selection of precursor molecules, substrates, and process conditions. For example, monolayers of molecules with a “stick-like” shape have high packing density and result in membranes with low permeance. On the other hand, molecules with a more complex 3-dimensional shape form less dense monolayers and thus membranes with higher permeance. A multilayer arrangement on the substrate is in principle not possible, because the precursor molecules have on one side a functional head group, which can form chemical bonds with corresponding chemical groups on the surface of the substrate. The functional group on the tail end of the precursor molecule is different, so that one molecule can only bind to the substrate but not to another molecule. In the case of multilayer physisorption (no chemical bonds) during molecular deposition, it is possible to remove excess molecules by simple rinsing leaving a real monolayer for further process steps.

After deposition of the self-assembled monolayer, the individual molecules have to be cross-linked with their neighbouring molecules. This cross-linking is started by radiation-induced breakage of bonds within the precursor molecules. In case bonds, which are spatially close to each other, opens within a short time frame in neighbouring molecules, these molecules can link with each other instead of individually relaxing. This statistical process transforms the monolayer, which originally had no internal connection, in a stable two-dimensional structure. The thickness of the membrane is compared to the lateral extension negligible.

In a last process step, the carbon nanomembrane can be released from the initial substrate and transferred to a suitable support structure in dependence on the final application. Without such a support, handling of a nanometre-thin membrane would be nearly impossible and large free-standing membranes would immediately rupture. However, CNMs on a mechanically stable, porous support with sufficiently small open areas can withstand pressure difference of several 10 bar.