Michael Fuhrer discusses the Secret Lives of Electrons in Atomically-thin Materials. Graphene is an atomically thin sheet of carbon atoms, and the basic constituent of graphite, the material found in pencil lead. Unlike in most materials, the electrons in graphene only travel in certain directions. They also travel at a constant speed independent of their energy, behaving as if they have no mass, like photons (particles of light). These hidden properties of graphene can explain why cylinders of graphene called carbon nanotubes can be metallic or semiconducting depending on their atomic structure.