This tiny US flag — just a few nanometers wide and invisible to the naked eye — is arguably the world’s smallest image of Old Glory, according to its creators at the University of Texas at Dallas. In an experiment, the nanoflag pattern emerged unexpectedly as sheets of the “stripe” material — molybdenum ditelluride — were heated to about 450 degrees Celsius, at which point its atoms began to rearrange and form new structures — the ‘stars’ in this false-color image. Each star consists of six central atoms of molybdenum surrounded by six atoms of tellurium. Stacked on top of one another, the stars form nanowires that might power advanced electronics. The transformation from stripes to stars is reported in the journal Advanced Materials. Credit: University of Texas at Dalla
For several years, a team at The University of Texas at Dallas has investigated various materials in search of those whose electrical properties might make them suitable for small, energy-efficient transistors to power next-generation electronic devices. They recently found one such material, but it was nothing anyone expected. Dr. Moon Kim and his colleagues describe a material that, when heated to about 450 degrees Celsius, transforms from an atomically thin, 2D sheet into an array of 1D nanowires, each just a few atoms wide. An image caught in mid-transformation looks like a tiny US flag, and with false colors added, it is the world’s smallest image of Old Glory.
Because the nanowires are semiconductors, they might be used as switching devices, just as silicon is used in today’s transistors to turn electric current on and off in electronic devices. “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.
In most cases, a phase diagram helps researchers predict structural and property changes in a material when it undergoes a phase transition. But nothing predicted what Kim’s team observed as it conducted experiments on molybdenum ditelluride. Using a transmission electron microscope, the researchers started with atomically thin, 2D 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.
When they increased 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 hexa-molybdenum hexa-telluride, a one-dimensional wire-like structure. The cross section of the new material is a structure consisting of six central atoms of molybdenum surrounded by six atoms of tellurium. As the phase transition progressed, part of the sample was still “stripes” and part had become “stars.” The team thought the pattern looked like a United States flag. They made a false-color version with a blue field behind the stars and half of the stripes colored red, to make a “nanoflag.”
When many of the individual nanowires are grouped together in bulk they behave more like a metal, which easily conducts 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.” http://www.utdallas.edu/news/2017/3/20-32477_Unexpected-Star-Spangled-Find-May-Lead-to-Advanced_story-wide.html?WT.mc_id=NewsHomePageCenterColumn
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