EuroWire September 2020

Transatlantic Cable

Research

The team has applied the intercalation technique to other elements, such as germanium, copper and gadolinium, but graphene is the focus of the research. In the case of gold, however, it was found that the intercalated atoms arranged themselves in a regular, periodically recurring two-dimensional (crystalline) structure along the silicon carbide surface. “If the intercalation is carried out at 600°C, the graphene layer prevents the gold atoms from agglomerating to form drops,” Mr Forti said of the function of the carbon layer in the sandwich structure. The production of a single-atom thickness of gold stirred interest in extremely thin materials and their potential characteristics. Research showed that the thin layer of gold develops its own electronic and semiconductor properties. As an example, the electrical conductivity of voluminous (three-dimensional) gold is nearly as good as that of copper but, as theoretical considerations forecast a metallic character for pure 2D gold, finding that a single-atom layer behaves like a semiconductor was surprising. “Interactions between the gold atoms and either the silicon carbide or the graphene carbon obviously still play a role here. This influences the energy levels of the electrons,” said Mr Starke. A second layer of gold atoms influences the electrical conductivity and gives a metallic character. “By varying the amount of sublimated gold, we can tightly control whether one or two layers of gold form,” said Mr Forti, offering the potential to use components with alternating single- and double-atomic gold layers. The new manufacturing method would need to be combined with common lithographic methods of chip production, but significantly smaller diodes could be produced. According to Mr Starke, the different electronic states of single- and double-layer gold could also be used in optical sensors. A further application idea results from effects caused by the intercalated gold in the adjacent graphene layer, depending on the thickness of the gold. “A gold layer one atom thick causes an n-doping in the graphene. This means we obtain electrons as charge carriers,” explained Mr Forti. In areas where the gold is two atomic layers thick, exactly the opposite – p-doping – occurs, where missing electrons or positively charged “holes” act as charge carriers. The gold also enhances the interaction of plasmons (fluctuations in the density of charge carriers) with electromagnetic radiation. “A structured, alternating arrangement of n- and p-doping in the graphene could [be used] as a highly sensitive yet high-resolution detector array for terahertz radiation, like those used in materials testing, for security checks at airports, or for wireless data transmission,” said Mr Starke. Mr Starke’s team has moved towards the production of two-dimensional precious metal layers, and carried out an intercalation experiment with silver – a strictly crystalline

New research into precious metals is pointing the way to new applications in microelectronics

Researchers from the Stuttgart-based Max Planck Institute for Solid State Research, working with partners in Pisa and Lund, have discovered that some precious metals (silver and gold, in particular) lose their conductive properties when applied in very thin layers, demonstrating that electrons behave differently in the two-dimensional layer of a material than in three-dimensional structures. The research teams, writing in Physical Review B and Nature Communications , believe that the discovery could lead to applications in microelectronics and sensor technology. By “very thin” the researchers refer to layers that are several hundred times thinner than gold leaf (itself only 0.1µm thick). Researchers Ulrich Starke and Stiven Forti have successfully created a gold layer just a single atom thick. Mr Starke is head of the Interface Analysis Facility at the Max Planck Institute, where his team is working on the border between voluminous (three-dimensional) and planar (two-dimensional) materials. Solid state researchers are interested in this transition because it is associated with changes in certain material properties – demonstrated in two-dimensional carbon, or graphene, where its electrons are significantly more mobile and allow the electrical conductivity to increase to 30 times that of the related three-dimensional graphite. Mr Starke explained, however, that producing layers of material just one atom thick is not easy. “With classical deposition methods, gold atoms, for example, would immediately agglomerate into three-dimensional clusters,” he said, so his team is working with a different method – intercalation. The researchers start with a silicon carbide wafer and, using a process they developed themselves, convert its surface into a single-atomic layer of graphene. “If we vaporise sublimated gold on to this silicon carbide-graphene arrangement in a high vacuum, the gold atoms migrate between the carbide and the graphene,” detailed Mr Forti, based at the Center for Nanotechnology Innovation in Pisa. It is not entirely understood how the thick gold atoms move into the interstitial space between the carbide and graphene, but the process is assisted by high temperatures.

Image: www.bigstockphoto.com Photographer Adrian Grosu

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September 2020