University of Maryland, College Park
A physicist peels back the layers of excitement about graphene.
Graphene is an atom-thick sheet of carbon in which electrons behave as if they have no mass. Atomic carbon layers have been grown epitaxially — that is, perfectly aligned with atoms in an underlying crystal surface — on metals and semiconductors for decades, so why the fuss lately?
Well, in the past few years much work in this field has revolved around graphene obtained by ‘exfoliating’ or peeling it from graphite. By mounting exfoliated graphene on insulating silicon dioxide, researchers observed a half-integer quantum Hall effect, an anomalous measurement that stems from the existence of a Landau level — the quantized orbit of electrons in a magnetic field — at exactly zero energy, a signature property of massless electrons.
But exfoliated graphene is dirty, lumpy and tiny (the biggest pieces are still a tenth of a millimetre in diameter). I wondered whether the older technique of epitaxial growth could produce a better material. In May, a group led by Joseph Stroscio showed that it could (D. L. Miller et al. Science 324, 924–927; 2009). Using scanning tunnelling spectroscopy to study epitaxial graphene on the surface of silicon carbide in a magnetic field, they showed that epitaxial graphene is extraordinarily clean and flat, and clearly exhibits the zero-energy Landau level. For the first time in any material, epitaxial graphene allows direct observation with atomic resolution of the behaviour of electrons in quantized Landau levels, opening a new window on the quantum Hall effect.
Researchers using other techniques, such as cyclotron resonance and photoemission, are also reporting an astonishingly clean electronic system in epitaxial graphene. Experiments on conductivity remain a challenge, but epitaxial graphene seems to have a bright future.