Invisible for Electrons

Scientists fabricate ultra-thin membranes

As thin as it gets: the carbon membranes recently created by Max Planck scientists are only one atom thick. For electrons, such membranes are almost completely transparent - using an electron microscope, scientists may thus be able to examine absorbed individual molecules on the membranes, and image the atomic structure of complex biological molecules. Such ultra-thin membranes may also be used to filter out gases (Nature, March 1, 2007).

Researchers at the Stuttgart-based Max Planck Institute for Solid State Research and the University of Manchester have created the thinnest membranes possible: They consist of only a single layer of carbon atoms, called graphene. Despite the thinness of the membranes, they are extremely stability. The reason for this is that the graphene membranes are not perfectly flat, but slightly corrugated - a form that gives the ultra-thin material stability - comparable with corrugated cardboard. "These two-dimensional membranes are completely different to ordinary three-dimensional crystals," says Dr. Jannik Meyer from the Max Planck Institute for Solid State Research. "We have just begun to explore the fundamental properties and possible applications."

Two years ago, scientists discovered a new class of thin materials that can be described as individual atomic planes pulled out of bulk crystals. These one-atom thick materials have rapidly become one of the most provocative topics in physics. However, it had remained doubtful whether such materials could exist without the support of a substratum.

Now, the research team headed by Dr. Jannik Meyer have produced such free-hanging membranes - specifically, from a single layer of carbon atoms called graphene. In order to fabricate graphene, only a pencil is principally needed: By rubbing ordinary graphite onto a surface, flakes of varying thickness break off from the layered material. Some layers are thereby formed that are only one atom thick. In order to find these and further process them, the scientists used a microfabrication method that is also used in the production of microprocessors. As a base, the researchers used a silicon crystal with an exactly calibrated oxide film; this was the only way that the researchers could make out the graphene mono-layer in the microscope by means of its very slight colour change. They then overlaid this with a metallic scaffold made from very fine gold wires having gaps between the wires 100 times smaller than the diameter of a strand of hair. In the next step, the researchers dissolved the silicon substratum in various acids. This permitted the graphene to hang freely on the scaffold. Fabricated in this manner, a graphene membrane between the gold wires has a surface of approximately one square micrometre, which is only a millionth of a square millimetre. However, this surface still contains 30 million carbon atoms that are all arranged on the free-hanging membrane.

These ultra-thin membranes may find use, for example, in filtering out gases, to make miniaturized ultra-fast electro-mechanical switches or as a non-obscuring support for electron microscopy to study individual molecules. "We have now demonstrated that extremely thin membranes that are only one atom thick can be produced. And we also believe that this technology can be adapted for use in real applications," says Prof. Andre Geim from the University of Manchester. "It still remains a challenge, however, to be able to fabricate these membranes economically and on a larger scale."