A drum for the nano orchestra
Cornell University's Paul McEuen and his colleagues built a tiny drum. To do this, they "stretched" two-dimensional layers of graphite over a frame made of silicon dioxide. Using laser light or applying an electrical voltage to a gold electrode, they were able to make the graphene "skin", which was only one layer thin, vibrate.
High-purity graphite consists of juxtaposed six-membered rings of carbon, stacked on top of each other with a spacing of 0.3 nanometers and held together only by van der Waals forces. The researchers peeled off these layers by brushing the material over silicon dioxide blocks in which they had previously etched micrometre-wide grooves. With the help of Raman spectroscopy, they determined the thickness of the layers above the cavities and were able to determine that in some cases they had actually succeeded in creating a single layer.
The eardrums vibrate at frequencies from one to 170 megahertz – for comparison: humans hear in the range of 18 to 18,000 hertz. For the thinnest layers, the frequency was more than ten times the theoretically expected value, which the researchers attribute to the stress in the graphene layer, which stretches to adhere to the substrate when it is wiped off. For a long time it was thought that individual graphene layers would immediately roll up into nanotubes. It was only in 2004 that graphene films could be produced. (af)