Gold electrodes rest on clumps of vanadium oxide wires that are each about 1,000 times smaller than a human hair. When baked in the presence of hydrogen gas, the wires next to the electrodes (dark region) absorb hydrogen and exhibit altered electronic behavior. Image: Jiang Wei/Rice University |
If you are not a condensed matter physicist, vanadium oxide
may be the coolest material you’ve never heard of. It’s a metal. It’s an
insulator. It’s a window coating and an optical switch. And 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? It’s an oxidized form of the
metal vanadium, an ingredient in hardened steel. When oxygen reacts with vanadium
to form vanadium oxide, 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 vanadium oxide 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 C, it goes through a phase transition that is both
electronic and structural,” said Rice’s Douglas Natelson, lead co-author of the
study in 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 an electrical conductor to
an electrical insulator.”
While other materials exhibit a similar electronic
about-face, vanadium oxide is unique in that the change occurs at a relatively
modest temperature—around 153 F—and sometimes at incredible speed—less than a
trillionth of second. In recent years, scientists have put these quirky
properties to work. In 2004, a group in London
used vanadium oxide to design a temperature-sensitive window coating that could
absorb sunlight on cold days and turn reflective on hot days. And electronics
researchers are also working to create optical switches from vanadium oxide.
“As an experimental physicist, vanadium oxide is intriguing
because the detailed physics of the material are still not well understood, and
theoretical models alone cannot give us the answers,” said Natelson, professor
of physics and astronomy and of electrical and computer engineering at Rice. “Experiments are key to understanding this.”
In 2010, Natelson and postdoctoral research associate Jiang
Wei began to systematically study the phase changes in vanadium oxide. Wei and
graduate student Heng Ji began by using a process called vapor deposition to
grow vanadium oxide wires that were about 1,000 times smaller than a human
hair. One set of experiments on wires that had been baked in the presence of
hydrogen gas returned particularly odd readings. Wei, Ji, and Natelson
determined that the hydrogen was apparently modifying the vanadium oxide
nanowires, but only those in contact with metal electrodes.
“The gold electrodes we were using to supply current to the
experiment were acting as a catalyst that split the hydrogen gas molecules into
atomic hydrogen, which could then diffuse into channels in the vanadium oxide,”
Natelson said. “It appears that the hydrogen is taken up into the vanadium
oxide crystals, and this changes their electronic properties. If a little
hydrogen is added, the phase transition happens at a slightly lower
temperature, and the insulating phase becomes more conductive. If enough
hydrogen is added, the transition to the insulating 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 vanadium oxide 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 vanadium oxide by adding other materials—a technique
known as doping. But Natelson said 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 said. “But the more
immediate payoff will likely be in helping us to better understand the physics
involved in the vanadium oxide 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.”
