Graphene — a single layer of graphite — is the structural unit that wraps to form carbon nanotubes and fullerenes. Because the electron conduction is nearly collision free, even at room temperature, this atomically thin carbon sheet is also interesting in its own right. Only recently have researchers figured out how to cleave a layer or layers of graphene from graphite, and even then, atomic force microscopy has been the only reliable method to determine the number of layers.

Now, Andrea Ferrari of Cambridge University and colleagues1 show that Raman spectroscopy provides a unique fingerprint of the number of graphene layers. Raman spectroscopy measures the energy shift of light in a solid that results from interactions with atomic vibrations, electrons or a combination of the two. In this study, it is shown that a single, strong peak in the spectrum of graphene — the so-called ‘2D’ peak — splits and shifts to a higher energy when there is more than one layer.

The fact that single and multiple layers of graphene have distinct Raman spectra reflects the difference in the electronic properties of a two-dimensional layer of carbon and three-dimensional graphite. This crossover appears to occur around five layers, when the spectrum of several layers of graphene and graphite become indistinguishable.