Inducing and enhancing gaps in graphene
In recent decades silicon technology has fuelled a computer revolution, with an exponential increase in the power of microprocessors and the size of memory units (commonly known as Moore's law). However silicon based technologies will eventually reach a limit where the tiny components on silicon chips become almost as small as individual atoms, and this could happen as soon as the end of the next decade. In order to develop the next generation of electronics a different approach and a new semiconductor material is needed. Graphene may be that material.
Graphite is made up of layers which can slide over each other. To make graphene, a single one of these layers must be isolated. Experimentalists have recently learned how to fashion quality samples of graphene but if computer chips are to be fabricated from graphene, then graphene layers must be placed on substrates of other materials. This theoretical project, funded as an EPSRC first grant (EP/H015655/1) has investigated how the electronic properties of graphene can be modified by sandwiching graphene layers between substrates and highly polarisable covering layers (superstrates). In such a sandwich, the vibrations in the superstrate (phonons) couple to electrons in the graphene, which can cause changes in the electronic structure.
Graphene can come in two main forms, monolayer graphene with a single atomically thick layer of carbon atoms, and bilayer graphene with two layers of carbon atoms. In monolayers, the substrate can affect the resultant electronics by making static structural changes in the graphene layer, opening a very small band gap which is not quite big enough to make transistors. As part of the project, simulations using three different sets of techniques have shown that vibrations of atoms in the covering layer can cause the small substrate induced gap to grow to sizes comparable with the silicon based semiconductors currently used in microchips [Read more in Ref. 5]. This is important for the manufacture of microchips because a gap is needed to make digital transistors. Moreover, the inclusion of high order terms in a perurbation theory increases the enhancement further, and can lead to the formation of spontaneous charge density wave (CDW) order for intermediate electron-phonon coupling. The CDW ordered state is inherently gapped and may be useful for digital transistors. [Read more in Ref. 1]
Bilayers are also affected by the substrate. There are small inherent energy differences due to the proximity of some carbon atoms in the different layers. These can be enhanced by an electron-phonon interaction, and when become sufficiently big a gap opens in the electronic structure of the bilayer, so that it becomes a semiconductor. This gap is distinct from the ones that are opened by placing an electric potential perpendicular to the bilayer graphene sheet. [Read more in Ref. 2]
The simulations demonstrate that there is an opportunity to use the substrate not just as a rigid scaffold for the graphene, but also as a means to enhance the properties of graphene for the design of advanced digital electronics. Results are also valid for other atomically thin materials such as boron nitride (BN) [see Refs 1 and 3].
- Enhancement of gaps in thin graphitic films for heterostructure formation J.P. Hague
arXiv:1303.6941, Accepted for publication in Phys. Rev. B (2014)
- Bias Free Gap Creation in Bilayer Graphene A.R. Davenport and J.P. Hague
arXiv:1308.1589, accepted for publication in J. Phys. Condens. Matter (2014)
- Gap modification of atomically thin boron nitride by phonon mediated interactions
Nanoscale Research Letters 7 303 (2012) [Open Access]
- Polarons in highly doped atomically thin graphitic materials.
J.P. Hague.Phys. Rev. B 86 064302 (2012) [arXiv:1107.2507] [pdf]
- Tunable graphene band gaps from superstrate-mediated interactions
Phys. Rev. B, 84 (2011) 155438 [arXiv:1103.3943] [pdf]