Large single-crystal graphene is possible, say scientists

A honeycomb-shaped monolayer of carbon atoms, uniform throughout the whole material, offers its exceptional properties to single-crystal graphene. On the other hand, polycrystalline graphene is formed by randomly oriented graphene islands, which decrease its quality. Currently, scientists are able to grow meter-sized polycrystalline graphene and smaller single-crystal graphene, ranging from 0.01 mm2 to a few ㎠. The synthesis of large single-crystal graphene at a low cost has been considered a critical goal of graphene synthesis.

In this study, graphene is grown on the surface of a 5 × 50 ㎠ copper foil, which was transformed into a single-crystal copper foil by heating to ~ 1,030℃. A temperature slope from hot to cold moved the so-called grain boundary onwards, creating a perfect single-crystal. In the heating and cooling treatment, copper atoms migrate inside the material, arranging into an ordered structure with fewer defects. "The secret to obtain single-crystal graphene of very large size, is to have a perfect single crystal copper as a base to start with. Large single-crystal copper foil is not available in the market, so labs must build it with their own means," explains Feng Ding, group leader at the Center for Multidimensional Carbon Materials.

Then, with another technique called chemical vapor deposition, millions of parallel graphene islands are formed on the copper foil surface. As more carbon atoms deposit on the foil, the islands keep on growing until they coalesce and form a very-close-to-perfect single-crystal graphene layer that covering the entire available surface.

In order to optimize the technique, the team had to consider four technical challenges: (i) preparation of single-crystal copper foil in a very large area, (ii) obtaining a high degree of alignment of the graphene islands following their nucleation and growth, (iii) seamlessly stitching the graphene islands into a single crystal through further growth and (iv) the fast growth of single-crystal graphene. Although previous reports have addressed some of the above challenges, this study overcame all of them and made the synthesis of meter-sized single-crystal graphene possible. The degree of the misaligned graphene islands is less than 0.1%, thus great reduced defects and grain boundaries in the products.

The current result was limited only by the size of the copper foil and, in principle, both the size of the copper foil and graphene film could perhaps be unlimited. In addition, considering the very short time of graphene synthesis (20 minutes) and the relatively low-cost experimental setup, the price of a single-crystal (or nearly single-crystal) graphene could be close to that of current polycrystalline graphene films.

"The dream of many scientists is to make it the material of the future, to replace silicon," enthuses Ding. "Now we are exploring which is the best material to grow graphene on top and how to use copper as a substrate for other interesting 2D materials."