#vanadium dioxide

Newswise — Many of today’s electronic devices — from the semiconductors in your cell phone to the photovoltaic cells in your solar panels — are built on thin-film substrates. The thin film is an electronically conductive material while the substrate is an inert material. Or is it? Physicists and materials scientists have long assumed substrates do not react to electrical stimulus, but new research from the University of California San Diego and a team of collaborators has shown that substrates are not inert after all. The discovery has the potential to help engineers build the dense, three-dimensional, brain-inspired computer chips needed for more energy-efficient computing. This work appears in Science. The research began four years ago in UC San Diego Associate Professor of Physics Alex Frañó’s lab. Frañó is a principal investigator and assistant director at the Quantum Materials for Energy-Efficient Neuromorphic Computing (Q-MEEN-C), one of the U.S. Department of Energy’s Energy Frontier Research Centers. One of the goals of Q-MEEN-C is to develop quantum materials that can be used in neuromorphic, or “brain-like” computing. 

Researchers have identified a metal that conducts electricity without conducting heat - an incredibly useful property that defies our current understanding of how conductors work.

The metal, found in 2017, contradicts something called the Wiedemann-Franz Law, which basically states that good conductors of electricity will also be proportionally good conductors of heat, which is why things like motors and appliances get so hot when you use them regularly.

But a team in the US showed this isn't the case for metallic vanadium dioxide (VO2) - a material that's already well known for its strange ability to switch from a see-through insulator to a conductive metal at the temperature of 67 degrees Celsius (152 degrees Fahrenheit).

"This was a totally unexpected finding," said lead researcher Junqiao Wu from Berkeley Lab's Materials Sciences Division back in January 2017.

"It shows a drastic breakdown of a textbook law that has been known to be robust for conventional conductors.  This discovery is of fundamental importance for understanding the basic electronic behaviour of novel conductors."

Not only does this unexpected property change what we know about conductors, it could also be incredibly useful - the metal could one day be used to convert wasted heat from engines and appliances back into electricity, or even create better window coverings that keep buildings cool.

Researchers already knew of a handful of other materials that conduct electricity better than heat, but they only display those properties at temperatures hundreds of degrees below zero, which makes them highly impractical for any real-world applications.

Vanadium dioxide, on the other hand, is usually only a conductor at warm temperatures well above room temperature, which means it has the ability to be a lot more practical.

To uncover this bizarre property, the team looked at the way that electrons move within vanadium dioxide's crystal lattice, as well as how much heat was being generated.

Surprisingly, they found that the thermal conductivity that could be attributed to the electrons in the material was 10 times smaller than that amount predicted by the Wiedemann-Franz Law.

The reason for this appears to be the synchronised way that the electrons move through the material.

"The electrons were moving in unison with each other, much like a fluid, instead of as individual particles like in normal metals," said Wu.

"For electrons, heat is a random motion.  Normal metals transport heat efficiently because there are so many different possible microscopic configurations that the individual electrons can jump between."

"In contrast, the coordinated, marching-band-like motion of electrons in vanadium dioxide is detrimental to heat transfer as there are fewer configurations available for the electrons to hop randomly between," he added.

Interestingly, when the researchers mixed the vanadium dioxide with other materials, they could 'tune' the amount of both electricity and heat that it could conduct - which could be incredibly useful for future applications.

For example, when the researchers added the metal tungsten to vanadium dioxide, they lowered the temperature at which the material became metallic, and also made it a better heat conductor.

That means that vanadium dioxide could help dissipate heat from a system, by only conducting heat when it hits a certain temperature.  Before that it would be an insulator.

Vanadium dioxide also has the unique ability of being transparent to around 30 degrees Celsius (86 degrees Fahrenheit), but then reflects infrared light above 60 degrees Celsius (140 degrees Fahrenheit) while remaining transparent to visible light.

So that means it could even be used as a window coating that reduces the temperature without the need for air conditioning.

"This material could be used to help stabilise temperature," said one of the researchers, Fan Yang.

"By tuning its thermal conductivity, the material can efficiently and automatically dissipate heat in the hot summer because it will have high thermal conductivity, but prevent heat loss in the cold winter because of its low thermal conductivity at lower temperatures."

A lot more research needs to be done on this puzzling material before it's commercialised further, but it's pretty exciting that we now know these bizarre properties exist in a material at room temperature.

The research was published in Science in 2017.

A version of this article was first published in January 2017.

Researchers have identified a metal that conducts electricity without conducting heat - an incredibly useful property that defies our current understanding of how conductors work.

The metal contradicts something called the Wiedemann-Franz Law, which basically states that good conductors of electricity will also be proportionally good conductors of heat, which is why things like motors and appliances get so hot when you use them regularly.

But a team in the US has shown that this isn't the case for metallic vanadium dioxide (VO2) - a material that's already well known for its strange ability to switch from a see-through insulator to a conductive metal at the temperature of 67 degrees Celsius (152 degrees Fahrenheit).

"This was a totally unexpected finding," said lead researcher Junqiao Wu, from Berkeley Lab’s Materials Sciences Division.

"It shows a drastic breakdown of a textbook law that has been known to be robust for conventional conductors. This discovery is of fundamental importance for understanding the basic electronic behaviour of novel conductors."

'Living' electrodes breathe new life into traditional silicon electronics

High-speed electronic devices that do not use much power are useful for wireless communication. High-speed operation has traditionally been achieved by making devices smaller, but as devices become smaller, fabrication becomes increasingly difficult. A research team at Osaka University is exploring another way to improve device performance: placing a patterned metal layer, i.e., a structural metamaterial, on top of a traditional substrate, e.g., silicon, to accelerate electron flow. The findings are published in the journal ACS Applied Electronic Materials. This method is promising, but a challenge is to make the structure of the metamaterial controllable, thereby allowing the properties of the metamaterial to be adjusted based on real-word conditions.

Powering the future—ultrathin films enhance electrical conductivity in flexible electronics

What if your electronic devices could adapt on the fly to temperature, pressure, or impact? Thanks to a new breakthrough in downsizing quantum materials, that idea is becoming a reality. In an article published this month in Applied Physics Express, a multi-institutional research team led by Osaka University announced that they have successfully synthesized an ultrathin vanadium dioxide film on a flexible substrate, in a way that preserves the film's electrical properties. Vanadium dioxide is well known in the scientific community for its ability to transition between conductor and insulator phases at nearly room temperature. This phase transition underpins smart and adaptable electronics that can adjust to their environment in real time. But there is a limit to how thin vanadium dioxide films can be, because making a material too small affects its ability to conduct or insulate electricity.

'Surprising' hidden activity of semiconductor material spotted by researchers

New research suggests that materials commonly overlooked in computer chip design actually play an important role in information processing, a discovery that could lead to faster and more efficient electronics. Using advanced imaging techniques, an international team led by Penn State researchers found that the material that a semiconductor chip device is built on, called the substrate, responds to changes in electricity much like the semiconductor on top of it. The researchers worked with the semiconductor material, vanadium dioxide, which they said shows great potential as an electronic switch. They also studied how vanadium dioxide interacts with the substrate material titanium dioxide and said they were surprised to discover that there seems to be an active layer in the substrate that behaves similarly to the semiconductor material on top of it when the semiconductor switches between an insulator—not letting electricity flow—and a metal—letting electricity flow.

Observing how light makes a metal—new details about the insulator-to-metal transition in a quantum material

With just the flick of a switch, quantum materials can undergo drastic changes. One notable example is the insulator-to-metal transition, a reversible physical phenomenon in which a material shifts from an insulating state, which will not conduct electricity, to a metallic one that will. The transition can be incredibly quick, an appealing property for emerging applications, including ultra-fast electronic and optical devices; brain-inspired neuromorphic computers; and energy-efficient smart windows that can adjust their transparency in response to changes in outside conditions. While fast insulator-to-metal transitions have been documented in many materials, how they unfold over time is much less well understood, as the combination of small size and quick speed made the transition challenging to observe with previously available experimental techniques.

Researchers Discover a Material With Brain-Like Learning Capabilities

Vanadium Dioxide has the ability to “remember” the entire history of past environmental stimuli.

During his research on phase transitions in Vanadium Dioxide (VO2), Mohammad Samizadeh Nikoo, a Ph.D. student at Ecole Polytechnique Fédérale de Lausanne’s (EPFL) Power and Wide-band-gap Electronics Research Laboratory (POWERlab), made an unexpected finding. When relaxed at room temperature, VO2 has an insulating phase and experiences a sharp insulator-to-metal transition at 68 °C (154 °F), where its lattice structure changes.

According to Samizadeh Nikoo, VO2 has a volatile memory: “the material reverts back to the insulating state right after removing the excitation.” He set out to find how long it takes for VO2 to change from one state to another for his thesis. However, his investigation took a different turn: after collecting hundreds of measurements, he discovered a memory effect in the material’s structure.