Published Oct. 28, 2010

The Microfabrication group (Group's web pages) at Aalto University, specialising in microfabrication and microfludics, has developed a new and rapid method for the fabrication of non-reflecting and self-cleaning surfaces. Surface properties are based on the nanostructured surface. The research results were just published in Advanced Materials, which is one of the most appreciated journals in the field of materials science.

The most laborious part the fabrication process was excluded when Aalto University's Microfabrication group developed a novel maskless method for the fabrication of pyramid-shaped nanostructures on a silicon surface using deep reactive ion etching. The nanostructured silicon wafer can be further used as a template to create an ealstomeric stamp, which can be used to replicate the original non-reflective and self-cleaning nanostructure into the different polymers.

Smooth silicon surfaces are mirror-like and they reflect more than 50 percent of incoming light, while nanostructured silicon and polymeric surfaces are almost completely non-reflecting. The reflectance is reduced at broad wavelength range due to smooth refractive index transition from air to substrate due to nanostructures, says Lauri Sainiemi from the Microfabrication group.

Non-reflecting surfaces and their fabrication methods are hot research topics because they are needed for the realization of more efficient solar cells. Similar nanostructured silicon and polymeric surfaces can also be utilized in chemical analysis, because low reflectance is needed in analysis procedures. The second beneficial property of the surfaces is self-cleaning, which is based on nanostructures, which are coated with a thin low surface energy film.

The applications of the developed nanofabrication methods for silicon and polymers ranges from sensors to solar cells. The biggest strength of the fabrication methods are their scalability and the possibility for large scale industrial manufacturing. I believe that there is interest because our fabrication methods enable simple and low-cost manufacturing of nanostructures on large areas and the methods are compatible with single-crystalline, poly-crystalline and amorphous silicon as well as a wide variety of different polymers, concludes Sainiemi.

The group has already develop surfaces for chemical analysis of drugs in collaboration with other research groups and that research will continue in the future. An interesting novel field is the development of more effective self-cleaning and dirt-repellant surfaces, that would especially benefit solar cell research. The fabrication of water-repellent surfaces is fairly straightforward, but liquids with low surface tension can still contaminate the surface. At the moment we are developing novel surfaces that also repel oily liquids.


Electron microscope image of a nanopatterend surfaces manufactured using the maskless method; the silicon surface, a stamp made from the silicon surface and the original nanopattern copied into two different polymers using the stamp. Both the original silicon surface and the copied polymer surfaces are self cleaning and do not reflect light.

The article was published on Oct. 27, 2010 in Advanced Materials on-line.
Non-Reflecting Silicon and Polymer Surfaces by Plasma Etching and Replication
Lauri Sainiemi, Ville Jokinen, Ali Shah, Maksim Shpak, Susanna Aura, Pia Suvanto, Sami Franssila. (DOI: 10.1002/adma.201001810)
http://onlinelibrary.wiley.com/doi/10.1002/adma.201001810/abstract