Researchers in the US have created the first artificial samples of graphene with electronic properties that can be controlled in a way not possible in the natural form of the material. The samples can be used to study the properties of so-called Dirac fermions, which give graphene many of its unique electronic properties. The work may also lead to the creation of a new generation of quantum materials and devices with exotic behaviour.
Graphene is a single layer of carbon atoms organized in a honeycomb lattice. Physicists know that particles, such as electrons, moving though such a structure behave as though they have no mass and travel through the material at near light speeds. These particles are called massless Dirac fermions and their behaviour could be exploited in a host of applications, including transistors that are faster than any that exist today.
The new “molecular” graphene, as it is has been dubbed, is similar to natural graphene except that its fundamental electronic properties can be tuned much more easily. It was made using a low-temperature scanning tunnelling microscope with a tip – made of iridium atoms – that can be used to individually position carbon-monoxide molecules on a perfectly smooth, conducting copper substrate. The carbon monoxide repels the freely moving electrons on the copper surface and “forces” them into a honeycomb pattern, where they then behave like massless graphene electrons, explains team leader Hari Manoharan of Stanford University.
Described by Dirac
“We confirmed that the graphene electrons are massless Dirac fermions by measuring the conductance spectrum of the electrons travelling in our material,” says Manoharan. “We showed that the results match the two-dimensional Dirac equation for massless particles moving at the speed of light rather than the conventional Schrödinger equation for massive electrons.”
The researchers then succeeded in tuning the properties of the electrons in the molecular graphene by moving the positions of the carbon-monoxide molecules on the copper surface. This has the effect of distorting the lattice structure so that it looks as though it has been squeezed along several axes – something that makes the electrons behave as though they have been exposed to a strong magnetic or electric field, although no actual such field has been applied. The team was also able to tune the density of the electrons on the copper surface by introducing defects or impurities into the system.
“Studying such artificial lattices in this way may certainly lead to technological applications, but they also provide a new level of control over Dirac fermions and allow us to experimentally access a set of phenomena that could only be investigated using theoretical calculations until now,” adds Manoharan. “Introducing tunable interactions between the electrons could allow us to make spin liquids in graphene, for instance, and observe the spin quantum Hall effect if we can succeed in introducing spin-orbit interactions between the electrons.”
He adds that molecular graphene is just the first of this type of “designer” quantum structure and hopes to make other nanoscale materials with such exotic topological properties using similar bottom-up techniques.
The work is reported in Nature 483 306.
