Simple Technique Transfers Clean Graphene Sheets

Graphene has a dirty little secret. When researchers build electronic devices with it, the standard process they use to move sheets of the delicate, single-atom-thick material into place can lead to contamination or damage that reduces device performance. But now, researchers in Taiwan have developed a simple and elegant way to transfer graphene that keeps the material clean (ACS Nano 2014, DOI: 10.1021/nn406170d).

Scientists produce high-quality, large-area sheets of graphene by growing them on copper surfaces using a method called chemical vapor deposition. Unfortunately, removing the graph­ene from the copper substrate is where the contamination problems start, says Chih-I Wu, an electrical engineer at National Taiwan University.

The most commonly used transfer method relies on a polymer-based material to hold the graphene in place while the metal below is etched away. The polymer, however, leaves behind a residue that contaminates the graphene. And cleaning it often damages the graphene so it doesn’t perform optimally, Wu says.

His team reports a cleaner approach that starts with submerging a copper substrate and its attached graph­ene in a petri dish filled with a metal-etching solution. The solution frees the graphene, which then floats on the surface. The researchers replace the etching solution with a mixture of isopropyl alcohol and water. Next, the researchers slip the device they’re building below the floating graphene sheet. The researchers then remove the alcohol-water mixture, lowering the graphene onto the device.

Using this transfer method, the researchers made a series of devices, including a transparent organic solar cell and graphene-based transistors. Electrical charges could move 50% faster through the transistors than in those made with graphene transferred by polymer.

Rodney S. Ruoff, a nanoengineer at the University of Texas, Austin, calls the approach interesting and useful. He suggests it could be used to make a type of low-power transistor called a BiSFET, which ideally needs very clean graphene.

The researchers plan to scale up the method and test the approach with 8- or 12-inch-diameter silicon wafers.