Mar 28, 2014
Achieving a Fermi level shift in graphene without an applied gate
A promising route towards nanodevice applications relies on the association of graphene with hexagonal boron nitride (hBN). Its insulating character and planarity allows graphene to reach high mobilities and improve performance. To achieve this, and reporting in Nanotechnology, researchers have analysed the role of point defects on the electronic properties of graphene/hBN compound structures.
(a) Moiré pattern formed by graphene and boron nitride; (b) Side and top views of the system after oxygen incorporation into boron nitride
Graphene/hBN systems present a lattice mismatch, or the so-called Moiré patterns (shown in figure 1a), and can be characterized by the rotation angle between the two layers. These patterns provide a periodic potential to graphene and lead to novel electronic properties at a relatively higher energy than the graphene neutrality point. This includes the appearance of new Dirac points, which represent regions of the dispersion relation in which electrons behave like massless Dirac fermions.
Energetically favourable defects
Now, researchers from the Department of Physics of the Universidade Federal de Minas Gerais in Brazil show that it may be energetically favourable to include defects into the BN substrate. Oxygen incorporation (see figure 1b), for example, is able to shift the Fermi level up to 1.0 eV relative to the isolated layer. Several defects are investigated, including some that are intrinsic to hBN and others that can be deliberately built. The analysis is based on first-principles calculations within the density functional theory (DFT) formalism as implemented in the SIESTA code, and includes a self-consistent treatment of the van der Waals interactions.
Introducing deliberate defects
Ion bombardment may produce boron vacancies and the calculations suggest that such defects may downshift the Fermi level 0.5 eV. This can lead to many interesting perspectives. For instance, ion implantation procedures could be used in graphene/hBN heterostructures to position the Fermi level at values that provide interesting electronic transport properties without the need for electrostatic doping. These interesting properties include Dirac points with anisotropic group velocities and novel effects due to electron-electron interactions in Van Hove singularities.
Full details can be found in the journal Nanotechnology 25 165705.
About the author
Matheus J S Matos is a PhD student in the Electronic Structure group at the Department of Physics of the Universidade Federal de Minas Gerais in Brazil. The work was conducted in collaboration with professors Mario S C Mazzoni and Hélio Chacham. The main interests of the group are related to structural, electronic and magnetic properties of nanostructures.