A one-atom-thick sheet of carbon known as graphene could show a new and useful type of magnetism. It could be generated by electrons hosted at sites where a carbon atom is missing. In the 5 March Physical Review Letters, Spanish researchers have shown with a scanning tunneling microscope that a lone electron exists at such a site, at least in graphite, which is similar to graphene. They suggest that electrons from many such sites could join forces to make the whole layer magnetic at relatively high temperatures, which may be useful for new types of electronic devices.
Researchers learned in 2004 how to make graphene, a single sheet of carbon atoms in a chicken-wire-like, hexagonal arrangement. Experiments confirmed rapid and unusual motion of electrons in these sheets. In addition, theorists have predicted that defects in the structure, such as vacancies–sites where an atom is missing–would make the sheet magnetic in an unusual way that could be useful for devices. Each vacancy would have dangling bonds–electrons from neighboring atoms that would effectively add up to a single electron, along with its unit of magnetism. But evidence for this picture has been indirect, as experiments have only measured the average effects of many vacancies, rather than focusing on a single vacancy.
To study a single vacancy, Ivan Brihuega of the Autonomous University of Madrid and his colleagues examined graphite, which consists of a stack of graphene sheets. They peeled off some layers from a graphite sample to reveal a nearly perfect surface. To create a handful of vacancies, the researchers knocked some atoms loose with a spray of argon ions. They then moved the sharp metal tip of a scanning tunneling microscope across the surface and measured the electrical conductance (inverse of resistance) for current flowing between the sample and the tip.
Away from the vacancy, the conductance varied smoothly as they changed the voltage on the tip, reaching zero at zero voltage and higher values at both positive and negative voltages. But when the tip was over the vacancy, the conductance dramatically increased at zero voltage. The researchers argue that this conductance peak represents an extra quantum state–essentially a molecular orbital–that holds a single electron. The voltage on the tip selects electrons with a specific energy, and zero voltage in this case selects the highest energy electrons in the sample. So a second electron trying to get to the vacancy-related state would be unable to overcome the repulsion of the first electron unless it had more energy than any other electron. The unpaired electron should also be magnetic, although the team did not measure this directly.
Although the results confirm expectations, they are still important, says Mikhail Katsnelson of Radboud University of Nijmegen in the Netherlands. “In a sense, all previous experimental evidence was indirect,” he says. “Now we know for sure that these states do exist, and we can go further and speculate about ferromagnetism.” Magnetism disappears above a certain critical temperature, which for many materials is too low to be practical. But theory suggests that for graphene, the critical temperature “can be much higher than in conventional magnetic semiconductors,” says Katsnelson. In this sense, layered carbon materials could be “a completely new generation of materials for spintronics,” the nascent field that aspires to exploit the magnetic properties of electrons for improved electronic devices.