Molecular graphene – the new designer structure?

According to researchers from Stanford University and the US Department of Energy's SLAC National Accelerator Laboratory, it is now possible to tune the fundamental properties of electrons so they behave in ways rarely seen in ordinary materials. The first examples are handcrafted, honeycomb shaped structures inspired by graphene. Initially, the electrons in the structure had graphene like properties. However, unlike ordinary electrons, they had no mass and travelled as if they were moving at the speed of light in a vacuum. The researchers were then able to tune these electrons in ways that are difficult to do in real graphene. Hari Manoharan, associate professor of physics at Stanford, describes the structure as 'molecular graphene'. To make the structure the scientists used a scanning tunnelling microscope to place individual carbon monoxide molecules on a perfectly smooth copper surface. The carbon monoxide repelled the free flowing electrons on the copper surface and forces them into a honeycomb pattern, where they behaved like graphene electrons. To tune the electrons' properties, the researchers repositioned the carbon monoxide molecules on the surface, thereby changing the symmetry of the electron flow. In some configurations, electrons acted as if they had been exposed to a magnetic or electric field, while in others researchers were able to finely tune the density of electrons on the surface by introducing defects or impurities. By writing complex patterns that mimicked changes in carbon-carbon bond lengths and strengths in graphene, the researchers were able to restore the electrons' mass in small, selected areas. "One of the wildest things we did was to make the electrons think they are in a huge magnetic field when, in fact, no real field had been applied," said Manoharan. The Stanford team calculated the positions where carbon atoms in graphene should be to make its electrons believe they were being exposed to magnetic fields ranging from zero to 60 tesla, more than 30% higher than the strongest continuous magnetic field ever achieved on Earth. The researchers then moved carbon monoxide molecules to steer the electrons into precisely those positions and the electrons responded by behaving exactly as predicted – as if they had been exposed to a real field. "Our new approach is a powerful new test bed for physics," said Manoharan. "Molecular graphene is just the first in a series of possible designer structures. We expect that our research will ultimately identify new nanoscale materials with useful electronic properties."