Hexagonal Copper Crystal Films Grow Highest-Quality Graphene Sheets

Graphene, a sheet of carbon just a single atom thick, has much potential in the future of electronics, but producing high-quality sheets for high- performance applications has been a challenge. Researchers at University of Illinois (Urbana-Champaign, IL) have discovered that the quality of graphene grown depends not on the surface finish, but on the crystal structure of the copper substrate on which it is grown.

Copper has been a popular substrate for graphene growth because it is inexpensive and promotes single layer growth. Graphene sheets are produced by pumping methane gas into a furnace with a sheet of copper foil. The carbon-hydrogen bonds break when the methane hits the copper, making the carbon atoms stick while the hydrogen floats away. Carbon atoms move around until they bond with each other in a single layer, resulting in graphene. Since copper foils are composed of a variety of different crystal structures, the resulting graphene is produced with many defects and multi-layer sections.

By combining data from several imaging techniques studying the graphene growth, the U of I team discovered that previous speculations that the roughness of the copper surface was the major factor in the quality of graphene were incorrect. Areas of the copper foil with hexagonally configured copper atoms resulted in the best graphene growth, with the carbon atoms growing laterally in a similar hexagonal pattern. And the copper foil sections with crystals in a cubic shape with wide gaps between atoms had the carbon atoms sticking in the holes and stacking vertically, growing the poorest quality graphene.

However, producing copper foil with only hexagonal copper crystals is difficult and prohibitively expensive, so the next challenge is finding a way to balance the value of the defect-free graphene with the cost of the single-layer copper crystal foil. One solution may be creating a foil with a higher percentage of hexagonal crystals, resulting in graphene that would be suitable for most applications. The team is also using this methodology to study the growth of other materials, including insulators to improve performance of graphene devices. The findings were published in the Nano Letters journal.