Microprocessing Technique Yields Ultrathin Membranes

Using a fabrication method for producing microprocessors, scientists have created what they said is the thinnest material that will ever exist: Carbon membranes that are only one atom thick. The membrane may allow them to examine individual molecules, image the atomic structure of complex biological molecules or filter gases.

Model of a graphene membrane only one atom thick. (Images: Max Planck Institute for Solid State Research)

Researchers at the Stuttgart-based Max Planck Institute for Solid State Research and the University of Manchester in England created the ultrathin membranes consisting of only a single layer of carbon atoms, called graphene. Despite their thinness they are extremely stable because the graphene membranes are not perfectly flat, but slightly corrugated -- a form that gives the material a stability comparable with corrugated cardboard.

"These two-dimensional membranes are completely different to ordinary three-dimensional crystals," said Jannik Meyer of 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 Meyer have produced such free-hanging membranes 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 and process them, the scientists used a microfabrication method that is also used in the production of microprocessors.

The thinnest material that will ever exist: The arrows point to a membrane that is as thin as a single atom. Here they hang on a scaffold of extremely fine gold wires. The black bar in the lower right corner represents a length of 500 nm.

As a base, the researchers used a silicon crystal with an exactly calibrated oxide film; using an electron microscope, the only way they could make out the graphene monolayer was by means of its very slight color change. They then overlaid this with a metallic scaffold made from very fine gold wires, with gaps between the wires 100 times smaller than the width of a strand of human hair.

In the next step, the researchers dissolved the silicon substratum in various acids. This allowed the graphene to hang freely on the scaffold. Fabricated in this manner, a graphene membrane between the gold wires has a surface of approximately 1 square µm, which is only a millionth of a square millimeter. However, this surface still contains 30 million carbon atoms that are all arranged on the free-hanging membrane.

According to the researchers, these ultrathin membranes may find use in filtering out gases, making miniaturized ultrafast electromechanical switches or as a nonobscuring 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," said professor 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."

The work is reported in the March 1 issue of Nature. For more information, visit: www.mpg.de/english/portal/index.html