We recently found a very interesting article titled “Star-spangled find may lead to advanced electronics” on Phys.org that we thought would be good to share with you.
Researchers at The University of Texas in Dallas for a few years now have been trying to find suitable materials to create energy-efficient transistors. And recently, they just stumbled into an unexpected find of such a material.
In the journal Advanced Materials, Dr. Moon and his colleagues described this material that transforms from a thin, “two-dimensional sheet into an array of one-dimensional nanowires,” when heated to about 450 degrees Celsius. The article was published online on March 10.
The image captured while the material is in its mid-transformation stage “looks like a tiny United States flag.”
“The phase transition we observed, this new structure, was not predicted by theory,” said Kim, the Louis Beecherl Jr. Distinguished Professor of materials science and engineering at UT Dallas.
As the nanowires are semiconductors, they can be potentially used as switching devices, just like how silicon is used currently to turn on and off electric current.
“These nanowires are about 10 times smaller than the smallest silicon wires, and, if used in future technology, would result in powerful energy-efficient devices,” Kim said. The lead authors of the study are Hui Zhu and Qingxiao Wang, graduate students in materials science and engineering in the Erik Jonsson School of Engineering and Computer Science.
Materials are subject to go through phase transitions when exterior conditions such as temperature or pressure changes, influencing the material’s atoms to rearrange and redistribute forming a different structure and composition. An example of this is liquid water which when cooled becomes ice or becomes gas (steam) when heated.
Scientists use “a type of graphic called a phase diagram” to help “researchers predict structural and property changes in a material when it undergoes a phase transition.”
But in Kim’s team’s case, there was nothing predicted as to what the team observed while they conducted experiments on the material called molybdenum ditelluride.
The researchers used a transmission electron microscope, and begin with an “atomically thin, two-dimensional sheets of molybdenum ditelluride, a material made up of one layer of molybdenum atoms and two layers of tellurium atoms. The material belongs to a class called transition metal dichalcogenides (TMDs), which show promise in replacing silicon in transistors.”
“We wanted to understand the thermal stability of this particular material,” Kim said. “We thought it was a good candidate for next-generation nanoelectronics. Out of curiosity, we set out to see whether it would be stable above room temperature.”
When the researchers turned up the temperature to above 450 degrees Celsius, two things happened.
“First, we saw a new pattern begin to emerge that was aesthetically pleasing to the eye,” Kim said. Across the surface of the sample, the repeating rows, or stripes, of molybdenum ditelluride layers began to transform into shapes that looked like tiny six-pointed stars, or flowers with six petals.
The material was transitioning into a one-dimensional wire-like structure, called hexa-molybdenum hexa-telluride. The cross-section of this new material shows a structure that consists of “six central atoms of molybdenum surrounded by six atoms of tellurium.”
It was observed that while the phase transition progressed, “part of the sample was still “stripes” and part had become “stars”.” The team thought it resembled the United States flag. They added colour to the pattern which perfectly fit the look of the US flag and called it a “nanoflag.”
“Then, when we examined the material more closely, we found that the transition we were seeing from ‘stripes’ to ‘stars’ was not in any of the phase diagrams,” Kim said. “Normally, when you heat up particular materials, you expect to see a different kind of material emerge as predicted by a phase diagram. But in this case, something unusual happened—it formed a whole new phase.”
The individual nanowires are semiconductors, which would allow electric current to move through and can be switched on and off. However, when the “nanowires are grouped together in bulk they behave more like a metal,” easily conducting current.
“We would want to use the nanowires one at a time because we are pushing the size of a transistor as small as possible,” Kim said. “Currently, the smallest transistor size is about 10 times larger than our nanowire. Each of ours is smaller than 1 nanometer in diameter, which is essentially an atomic-scale wire.
“Before we can put this discovery to use and make an actual device, we have many more studies to do, including determining how to separate out the individual nanowires, and overcoming technical challenges to manufacturing and mass production,” Kim said. “But this is a start.”
To read the source article, click here.