Using density functional theory and measurement data from ARPES, the HZB team investigated the origin of the repeating Au(111) bands and resolved them as deep surface resonances. These resonances lead to an onion-like Fermi surface of Au(111). Image: HZB.Graphene consists of carbon atoms that crosslink in a plane to form a flat honeycomb structure. In addition to a surprisingly high mechanical stability, graphene has exciting electronic properties: its electrons behave like massless particles, which can be clearly demonstrated in spectrometric experiments. Measurements reveal a linear dependence of energy on momentum, in the form of so-called Dirac cones – two lines that cross without a band gap, meaning without an energy difference between electrons in the conduction band and those in the valence bands.
Artificial variants of graphene architecture are a hot topic in materials research right now. Researchers have tried replacing the carbon atoms with quantum dots of silicon and trapping ultracold atoms in the honeycomb lattice with strong laser fields. They have also pushed carbon monoxide molecules into place on a copper surface piece-by-piece with a scanning tunneling microscope, imparting characteristic graphene properties to the electrons in the copper.
A recent study suggested that it is infinitely easier to make artificial graphene using C60 molecules called buckyballs. This revealed that just a single uniform layer of buckyablls needs to be vapor-deposited onto gold for the gold electrons to take on the special properties of graphene. Measurements of photoemission spectra appeared to show a kind of Dirac cone.
"That would be really quite amazing," says Andrei Varykhalov from Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in Germany, who heads a photoemission and scanning tunneling microscopy group. "Because the C60 molecule is absolutely nonpolar, it was hard for us to imagine how such molecules would exert a strong influence on the electrons in the gold." So Varykhalov and his team at HZB launched a series of measurements to test this hypothesis.
In tricky and detailed analyses, the HZB team was able to study C60 layers on gold over a much larger energy range and for different measurement parameters. They used angle-resolved photoemission spectroscopy (ARPES) at BESSY II, which can make particularly precise measurements, and also analyzed the electron spins.
"We see a parabolic relationship between momentum and energy in our measured data, so it's a very normal behavior. These signals come from the electrons deep in the substrate (gold or copper) and not the layer, which could be affected by the buckyballs," says Maxim Krivenkov, lead author of a paper on this work in Nanoscale.
The team was also able to explain the linear measurement curves from the previous study. "These measurement curves merely mimic the Dirac cones; they are an artifact, so to speak, of a deflection of the photoelectrons as they leave the gold and pass through the C60 layer," Varykhalov explains. Therefore, the buckyball layer on gold cannot be considered an artificial graphene.
This story is adapted from material from HZB, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.