Optically transparent conductive materials are essential components for making electrical contacts in touch-sensitive screens, liquid crystal displays and solar cells. Graphene is a planar honey-combed structured carbon sheet which shows potential for these applications. However, the production of large areas of high quality graphene is a challenge.

Current approaches to graphene synthesis include mechanical or chemical peeling of three-dimensional graphite crystals. However, repeated peeling introduces structural defects into the graphene sheets. Other methods based on the high-temperature decomposition of silicon carbide are only able to produce small samples.

Now, Byunh Hee Hong at Sungkyunkwan University, Korea, and collaborators1 have devised a method to grow graphene films on a large scale by depositing carbon onto substrates covered with a thin layer nickel.

The researchers first deposited a thin 300-nanometer nickel (Ni) film on a silicon oxide support. Next, the Ni film was exposed to a hot gaseous mixture of methane/hydrogen, which generated graphene films containing up to ten sheets.

Fig. 1: Future applications of graphene-based transparent electrodes.

The graphite was thinned down to graphene by controlling the initial thickness of the Ni film, growth temperature, and ratio and concentration of the gases.

“Actually, the only difference between graphene and graphite is the number of layers,” says Hong. “The roughness and thickness of nickel films are very important parameters for the growth of thinner and better graphene layers.”

This optimised multilayer deposition led to easy-to-transfer, rippled graphene films with exceptional electrical conductivity and optical transparency. “The nickel layer shrinks more than graphene while cooling down, so the ripple structures are naturally formed to release the stress,” says Hong. Notably, the ripples moderately compensate the strain exerted on the films on stretching, thereby enhancing their stretchability.

The researchers developed two processes to isolate the films. In a wet-transfer process, they soaked the supported films into mild etchants to slowly dissolve the nickel layers and lift the graphene off the support. In the second process, they attached them to soft substrates before etching the metal away. This dry-transfer process allows the film size and shape to be tailored without requiring any additional lithography, providing an especially attractive avenue for graphene-based device production.