Fullerenes are molecules made up of carbon atoms arranged in a sphere and look like hollow cages. The most common fullerene is carbon-60 – also called a "buckyball" – but fullerenes containing more than 60 carbon atoms also exist. Endohedral fullerenes are fullerenes that have trapped atoms, ions or clusters inside the cage structure. These atoms can be metallic, in which case the molecules are called metallofullerenes. Cages of metallofullerenes usually consist of 80, 82, 84 or even higher numbers of carbon atoms.
Alexey Popov of the Leibniz Institute for Solid State and Materials Research in Dresden and Moscow State University, together with colleagues in Dresden and the University of Science and Technology in Hefei, studied a nitride clusterfullerene containing titanium and scandium (its formula is TiSc2N@C80). The molecule studied is a special type of fullerene in which the carbon cage (C80) encapsulates a nitride-metal cluster, TiSc2N.
The researchers found that if one metal in such compounds is a transition metal (titanium), the spin density of electrons is localized on this metal. Spin density is defined as the total electron density of electrons of one spin (spin "up", for example) – minus the total electron density of the electrons of the other spin (spin "down").
The team also discovered that, by varying the redox state of the whole molecule (via electrochemistry techniques, as in this work), the oxidation state of the titanium atom can actually be changed. "This is the first endohedral fullerene in which redox activity is exclusively due to the encaged metal atoms," explained Popov. "In most other cases, the fullerene cage itself is responsible for redox activity."
The result means that the electronic state of endohedral species in fullerenes could now be precisely tuned.
"Spin flow"
And that is not all: the endohedral cluster must be rotating inside the fullerene cage, which leads to the very flexible electron spin density distribution in TiSc2N@C80. It means that the spin density distribution also changes quickly – something that the researchers have dubbed "spin flow". "We can follow the spin populations using molecular dynamics simulations and obtain a 'spin-flow vibrational spectrum' that clearly shows what kinds of internal motion are coupled to the spin flow," Popov told nanotechweb.org.
Although spin-flow vibrational spectroscopy remains a theoretical tool for the moment, it shows what kinds of vibration are important for spin transport in molecules. According to the researchers, all types of molecules – not just endohedral fullerenes – might be studied using this technique. It could be useful for studying spin transport in molecular devices and might even be important for exploring the magnetic properties of materials as well developing spintronics devices that exploit both the charge on an electron and its spin.
The Dresden-Moscow-Hefei team will now try to obtain the same spectra in real experiments. It will also study other molecules to find out how general the method might be.
The work was published in ACS Nano.