Since the development of graphene and the discovery that its electrons act as Dirac fermions, many other 2D materials have been investigated. Like graphene, it has long been a wonder as to whether these materials also contain the same linear electronic state dispersions at the corners of the Brillouin zone (BZ), that give rise to Dirac cones. A team of international researchers has now provided compelling experimental evidence that Dirac cones can be found in silicene (2D silicon layers), at high symmetry points of the BZ with the apex just below the Fermi level.
The discovery of graphene has paved the way in terms of 2D research, and in recent years many other 2D materials have come to fruition. However, being present as 2D sheet doesn’t necessarily mean that the material will possess the same properties that graphene does, namely the presence of a Dirac cone. The theory predicts that only a handful of materials, including silicene (2D silicon), should possess a Dirac cone.
Whilst other materials in theory possess a Dirac cone, graphene has been the only material to have it proved experimentally. With silicon occupying the same periodic group as carbon, there has been much excitement of the potential of experimentally finding such phenomena in honeycomb-like silicene sheets.
Silicene is produced by epitaxial growth onto crystalline surfaces, commonly silver. There has also been much wonder as to whether silicene would preserve its Dirac point when it is only grown on metallic substrates. Other scenarios troubling researchers for a long time has been whether the electronic structures of silicene and the metallic substrate can be separated, and is the Dirac cone actually observable?
Recently, it was thought that this phenomenon had been observed, but the results turned out to be due to strong silver-silicon interactions at the interface, with the linear dispersions being attributed to the silver, not silicene. The disproving of these results prompted this team of researchers to attempt to find thesilicene Dirac cone on a gold substrate.
The researchers found that due to possessing a lower surface energy than gold, silicene was favourably formed on the (111) crystallographic plane of the gold substrate. The researchers used low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and high resolution angle resolved photo-emission spectroscopy (HR-ARPES)to deduce the formation of an extended 2D silicon sheet with long-range ordering.
Scanning tunnelling microscopy (STM) showed conclusive evidence of a Dirac cone at high symmetry points. The researchers also found that the apex of the Dirac cone was present 0.5 eV below the Fermi level, at the K-point- the point in the middle of an edge that joins two of the hexagonal faces. What provides conclusive evidence, though, is the presence of a single component in the Si 2p core. Whilst it does not sound like much, it shows that only a single silicon atom was involved in the measurement, thus negating the possibility of an electronic crossover between the substrate and silicon and proving that the Dirac cone was located on the Silicene sheet.
The interactions between the gold substrate and the silicene sheet are weakly bound and isone of the main reasons why silicene could be experimentally observed, unlike silver which forms strong interactions and electronic crossovers. Freestanding silicene has zero band gap and the weak interactions open-up the band gap, allowing the Dirac cone to be observed. The Au-Au bonds can also be ruled out as there was no band gap opening recorded for these bonds. The researchers also found compelling evidence that the 2D overlayer is responsible for a linear dispersion in the valence bad with a Fermi velocity of 106 ms-1, which is comparable to graphene.
Surface characterisation showed extended and homogeneous domains on the silicene sheet and it suggested that silicene may now offer a route to the fabrication of novel opto-electronic devices.
Saddedine S., Enriquez H., Bendounan A., Das P. K., Vobornik I., Kara A., Mayne A. J., Sirotti F., Dujardin G., Oughaddou H., Compelling experimental evidence of a Dirac cone in the electronic structure of a 2D Silicon layer, Scientific Reports, 2017, 7, 44400