Reversible doping: hydrogen alters ‘weird’ metal

It’s a metal, an insulator, a window coating, and an optical switch. Now, thanks to a new study by physicists at Rice University, scientists have a new way to reversibly alter vanadium oxide’s electronic properties by treating it with one of the simplest substances—hydrogen. So what is vanadium oxide (VO2)? It’s an oxidized form of the metal vanadium, an ingredient in hardened steel. When oxygen reacts with vanadium to form VO2 , the atoms form crystals that look like long rectangular boxes. The vanadium atoms line up along the four edges of the box in regularly spaced rows. A single crystal of VO2 can have many of these boxes lined up side by side, and the crystals conduct electricity like wire as long as they are kept warm. “The weird thing about this material is that if you cool it, when you get to 67 degrees Celsius, it goes through a phase transition that is both electronic and structural,” says Douglas Natelson, lead co-author of the study in this week’s Nature Nanotechnology. “Structurally, the vanadium atoms pair up and each pair is slightly canted, so you no longer have these long chains. When the phase changes, and these pairings take place, the material changes from being a electrical conductor to an electrical insulator.” ing phase disappears altogether.” To gain insight into just how the hydrogen is able to alter the transition, the experimenters consulted with theoretical physicist Andriy Nevidomskyy, assistant professor of physics and astronomy at Rice. Nevidomskyy’s calculations showed that the hydrogen changes the amount of charge in the VO2 material and also forces the crystal to expand slightly. Both of these effects favor the metallic state. This is not the first time physicists have lowered the transition temperature of VO2 by adding other materials—a technique known as “doping.” But Natelson says Rice’s hydrogen doping is unique in that it is completely reversible: to remove the hydrogen, the material simply has to be baked in an oven at moderate temperature. “On the applied side, there may be a number of applications for this, like ultrasensitive hydrogen sensors,” Natelson says. “But the more immediate payoff will likely be in helping us to better understand the physics involved in the VO2 phase transition. “If we can find out exactly how much hydrogen is required to shut down the transition, then we will have a knob that we can turn to systematically raise or lower the temperature in future experiments.” The research was funded by the Department of Energy’s Office of Science and through a fellowship from the Robert A. Welch Foundation.(futurity)