Transistor made from vanadium dioxide could function as smart window for blocking infrared light

February 8, 2013
Figure 1:Below 47 °C, the vanadium ions in a film of VO2 pair up into a different crystal structure and the electrons no longer conduct freely. The transition can be reversed with a positive voltage, applied at the top of the film. The voltage induces electrons to move to the region near the surface, which restores the high-temperature structure and metallic behavior throughout the entire film. © 2012 Nature Publishing Group

The transistor is the ultimate on-off switch. When a voltage is applied to the surface of a semiconductor, current flows; when the voltage is reversed, current is blocked. Researchers have tried for decades to replicate these effects in transition metal oxides by using a voltage to convert the material from an insulator to a metal, but the induced change only occurs within a few atomic layers of the surface.

Now, Masaki Nakano and colleagues at the RIKEN Advanced Science Institute in Wako have discovered that applying a voltage to a vanadium dioxide (VO2) film several tens of nanometers thick converts the entire film from an insulator to a metal. The findings point to the specific material properties needed to make such devices work. They may also lead to new types of 'smart' technology.

The of can be tuned by changing their or temperature. For example, VO2 is an insulator at room temperature, but heating it or replacing a small fraction of the vanadium atoms with tungsten (an ) causes a phase transition where the vanadium ions, which are paired up at low temperature, unfasten into a different crystal structure in which electrons are mobile. In principle, applying a positive voltage to the surface of an insulating VO2 film can accomplish the same effect by inducing electrons to the surface, making this region metallic.

Researchers have assumed that this charging effect would be limited to a few just below the surface because the excess of electrons cancels out the applied electric field (an effect called screening). But Nakano and his colleagues found that the excess electrons were enough to 'trigger' the change associated with metallic behavior (Fig. 1). "The surface lattice distortion propagates through the entire film, followed by an electronic phase transition inside the bulk region," he says. The voltage-induced transition decreases VO2's resistance by a factor of 100.

The team is actively seeking other materials like VO2, as well as technological applications. One is a heat switch. Since temperature determines if VO2 is a metal or an insulator, it also determines the frequency of light the material absorbs. VO2-coated glass could therefore act as a 'smart window', passing or blocking infrared light depending on the temperature outside. "Normally, this switching temperature is fixed," says Nakano. "Our device adds electrical switching functionality to a smart window, which is very promising for energy-saving applications."

Explore further: Putting a Strain on Nanowires Could Yield Colossal Results

More information: 1.Nakano, M., Shibuya, K., Okuyama, D., Hatano, T., Ono, S., Kawasaki, M., Iwasa, Y. & Tokura, Y. Collective bulk carrier delocalization driven by electrostatic surface charge accumulation. Nature 487, 459–462 (2012). www.nature.com/nature/journal/v487/n7408/abs/nature11296.html

Related Stories

Putting a Strain on Nanowires Could Yield Colossal Results

September 17, 2009

(PhysOrg.com) -- In finally answering an elusive scientific question, researchers with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have shown that the selective placement of strain ...

Bending light with better precision

August 15, 2011

Physicists from the University of California at San Diego (UCSD) have demonstrated a new technique to control the speed and direction of light using memory metamaterials whose properties can be repeatedly changed.

Reversible doping: Hydrogen flips switch on vanadium oxide

May 21, 2012

If you are not a condensed matter physicist, vanadium oxide (VO2) 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 ...

Recommended for you

Seeing quantum motion

August 28, 2015

Consider the pendulum of a grandfather clock. If you forget to wind it, you will eventually find the pendulum at rest, unmoving. However, this simple observation is only valid at the level of classical physics—the laws ...

A little light interaction leaves quantum physicists beaming

August 24, 2015

A team of physicists at the University of Toronto (U of T) have taken a step toward making the essential building block of quantum computers out of pure light. Their advance, described in a paper published this week in Nature ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.