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The semiconducting cousin of graphene

Electronics is based on semiconductors, that is on materials with an energetic separation (band gap) between the valence and the conduction band. The bandgap allows to control the flow or not of current, i.e. to define the logical states of electronic devices. Despite all the exciting electronic properties of graphene, it does not exhibit a bandgap. In collaboration with GeorgiaTech, researchers from LPS and SOLEIL have demonstrated that an atomic layer with the same honeycomb structure as graphene – the buffer layer – allows to obtain a gap of more than 500 meV.

Electronics is based on semiconductors. The raison for that is the energy barrier (bandgap) that electrons must overcome to conduct along the material. This energy barrier can be overcome with an electric field, allowing or not a current flow through the material. In this way, the states 0 (no current) and 1 (current flow) are determined, which are the basis of all our electronic devices. Graphene has many exceptional electronic properties like ballistic transport, but it lacks of a sizeable energy gap. Indeed, its valence and conduction bands, known as π and π∗ bands, meet themselves, so graphene is said to be a zero-gap material, a problem for developing standard electronics.

In the context of a long-term collaboration with GeorgiaTech, researchers from LPS and SOLEIL have studied the electronic properties and the atomic structure of graphene grown on silicon carbide. Here, graphene is grown by annealing, inducing the sublimation of Si atoms on the last atomic layers. When graphene is grown on the Si-face of the SiC, the remaining carbon after annealing first organizes itself into the "buffer layer", an atomic layer with the characteristic honeycomb structure of graphene but without its electronic properties. Afterwards, graphene grows on top of this buffer layer.

Nevius et al. have focused on the electronic properties of the buffer layer by using angle-resolved photoemission spectroscopy (ARPES) [1,2]. This technique allows to measure the valence band of the materials by illuminating samples with photons and detecting the emission angle and the kinetic energy of the photoexcited electrons. By doing so, it has been observed the largest bandgap observed on epitaxially grown graphene samples (greater than 0.5 eV). What is more, the gap is located at the Fermi level, an important requirement for graphene becoming a real applicative material.

References :

[1] M. S. Nevius, M. Conrad, F. Wang, A. Celis, M. N. Nair, A. Taleb-Ibrahimi, A. Tejeda, and E. H. Conrad.
Semiconducting Graphene from Highly Ordered Substrate Interactions
Physical Review Letters 115, 136802 (2015).
[2] A. Lanzara, Physics 8, 91 (2015).

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Antonio Tejeda