By Christine Middleton
Vanadium dioxide undergoes a metal–insulator transition that can be used for switching in ultrafast electronic devices. At low temperatures, the compound adopts an insulating monoclinic crystal structure, but around room temperature, it morphs into a conducting rutile structure. If the structural and electronic transitions could be decoupled, switching between conducting and insulating states would be faster.
Chang-Beom Eom at the University of Wisconsin–Madison, Jaichan Lee at Sungkyunkwan University in South Korea, and coworkers have achieved that goal by devising a way to maintain the insulating crystal structure in the conducting state. They used a VO2 bilayer in which the metal–insulator transition temperature of one layer was lowered by adding point defects. That process introduced a temperature range in which one layer was in the conducting rutile state while the other was still in the monoclinic state (left graph). Despite half the crystal still being monoclinic, the material conducted as though the entire structure had switched to the rutile phase (right graph). The monoclinic layer was also conducting, even though it retained the insulating crystal structure.
A theoretical model of the bilayer showed that the single-step resistivity transition occurred only in a finite range of structure thickness. For thin bilayers, interfacial energy between the two layers dominated the bulk energy, so having two conducting layers was energetically favorable. However, if the monoclinic layer was thicker than 9.4 nm, bulk energy was dominant, and the layer behaved as an insulator as soon as its structure changed. Now that Eom and his colleagues understand how to stabilize the conducting monoclinic state, they can study the crystal’s electronic properties independent of the underlying lattice structure.