Tag Archives: artificial graphene

Artificial graphene with buckyballs

A July 21, 2022 news item on Nanowerk describes graphene in its ‘natural’ state and explains what ‘artificial’ graphene is although there is no mention of why variants are a hot topic,

Graphene consists of carbon atoms that crosslink in a plane to form a flat honeycomb structure. In addition to surprisingly high mechanical stability, the material has exciting electronic properties: The electrons behave like massless particles, which can be clearly demonstrated in spectrometric experiments.

Measurements reveal a linear dependence of energy on momentum, namely the so-called Dirac cones – two lines that cross without a band gap – i.e. an energy difference between electrons in the conduction band and those in the valence bands.

Variants in graphene architecture

Artificial variants of graphene architecture are a hot topic in materials research right now. Instead of carbon atoms, quantum dots of silicon have been placed, ultracold atoms have been trapped in the honeycomb lattice with strong laser fields, or carbon monoxide molecules have been pushed into place on a copper surface piece by piece with a scanning tunneling microscope, where they could impart the characteristic graphene properties to the electrons of the copper.

A July 21, 2022 Helmholtz-Zentrum Berlin (HZB) press release (also on EurekAlert), which originated the news item, describes research into whether or not layering buckyballs onto gold would result in artificial graphene,

Artificial graphene with buckyballs?

A recent study suggested that it is infinitely easier to make artificial graphene using C60 molecules called buckyballs [or buckminsterfullerenes or, more generically, fullerenes]. Only a uniform layer of these needs to be vapor-deposited onto gold for the gold electrons to take on the special graphene properties. Measurements of photoemission spectra appeared to show a kind of Dirac cone.

Analysis of band structures at BESSY II

“That would be really quite amazing,” says Dr. Andrei Varykhalov, of HZB, 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 launched a series of measurements to test this hypothesis.

In tricky and detailed analyses, the Berlin team was able to study C60 layers on gold over a much larger energy range and for different measurement parameters. They used angle-resolved ARPES spectroscopy at BESSY II [third-generation synchrotron radiation source], which enables particularly precise measurements, and also analysed electron spin for some measurements.

Normal behavior

“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,” explains Dr. Maxim Krivenkov, lead author of the study. 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.

Caption: Measurement data from BESSY II before and after deposition of C60 molecules demonstrate the replication of the band structure and the emergence of cone-like band crossings. A scanning electron microscopy of the buckyballs on gold is superimposed in the centre. Credit: HZB

Here’s a link to and a citation for the paper,

On the problem of Dirac cones in fullerenes on gold by M. Krivenkov, D. Marchenko, M. Sajedi, A. Fedorov, O. J. Clark, J. Sánchez-Barriga, E. D. L. Rienks, O. Rader and A. Varykhalov. Nanoscale, 2022,14, 9124-9133 First published: 23 May 2022

This paper is open access.

Artificial graphene?

I’m not sure I ever want to hear the word ‘revolutionary’ or its cousin’ revolution’ in relationship to science and/or technology ever again and I don’t think anyone’s going to be paying attention to this heartfelt plea: please, please, please find another word for a couple of years at least.  That said, artificial graphene does sound exciting as it’s described in a Feb. 17, 2014 news item on Azonano,

A new breed of ultra thin super-material has the potential to cause a technological revolution. “Artificial graphene” should lead to faster, smaller and lighter electronic and optical devices of all kinds, including higher performance photovoltaic cells, lasers or LED lighting.

For the first time, scientists are able to produce and have analysed artificial graphene from traditional semiconductor materials. Such is the scientific importance of this breakthrough these findings were published recently in one of the world’s leading physics journals, Physical Review X. A researcher from the University of Luxembourg played an important role in this highly innovative work.

The University of Luxembourg Feb. 14, 2014 news release (also on EurekAlert), which originated the news item, describes both graphene and artificial graphene

Graphene (derived from graphite) is a one atom thick honeycomb lattice of carbon atoms. This strong, flexible, conducting and transparent material has huge scientific and technological potential. Only discovered in 2004, there is a major global push to understand its potential uses. Artificial graphene has the same honeycomb structure, but in this case, instead of carbon atoms, nanometer-thick semiconductor crystals are used. Changing the size, shape and chemical nature of the nano-crystals, makes it possible to tailor the material to each specific task.

University of Luxembourg researcher Dr. Efterpi Kalesaki from the Physics and Materials Science Research Unit is the first author of the article appearing in the Physical Review X . Dr. Kalesaki said: “these self‐assembled semi-conducting nano-crystals with a honeycomb structure are emerging as a new class of systems with great potential.” Prof Ludger Wirtz, head of the Theoretical Solid-State Physics group at the University of Luxembourg, added: “artificial graphene opens the door to a wide variety of materials with variable nano‐geometry and ‘tunable’ properties.”

I’m going to provide two links and two citations to the paper as its publishing journal is currently beta testing a new website and the paper is available on both,

Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry by E. Kalesaki, C. Delerue, C. Morais Smith, W. Beugeling, G. Allan, and D. Vanmaekelbergh. Phys. Rev. X 4, 011010 (2014) [12 pages] DOI: 10.1103/PhysRevX.4.011010

Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry by E. Kalesaki, C. Delerue, C. Morais Smith, W. Beugeling, G. Allan, and D. Vanmaekelbergh. Phys. Rev. X 4, 011010 – Published 30 January 2014 DOI: http://dx.doi.org/10.1103/PhysRevX.4.011010

The second link to the paper will take you to the journal’s beta site. I have to give the designers a big thumbs up on the new design. To contextualize my review, I’m not a fan of changing website designs as functionality is too often sacrificed for ‘good looks’. Sadly, I do have a bit more work cutting and pasting with the new version but I’m hugely relieved that I did not have to spend several minutes trying to find the information.

Both versions of the paper are open access.