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Posted: Dec 27, 2016
Researchers use world's smallest diamonds to make wires 3 atoms wide
(Nanowerk News) Scientists at Stanford University and the Department of Energy's SLAC National Accelerator Laboratory have discovered a way to use diamondoids - the smallest possible bits of diamond - to assemble atoms into the thinnest possible electrical wires, just three atoms wide.
An illustration shows the basic nanowire building block – a diamondoid cage carrying atoms of copper and sulfur – drifting toward the growing tip of a nanowire, center, where it will attach in a way determined by its size and shape. The copper and sulfur atoms wind up on the inside, forming a core of semiconducting material, and the diamondoids remain on the outside, where they function as an insulating shell. (Image: SLAC National Accelerator Laboratory)
"What we have shown here is that we can make tiny, conductive wires of the smallest possible size that essentially assemble themselves," said Hao Yan, a Stanford postdoctoral researcher and lead author of the paper. "The process is a simple, one-pot synthesis. You dump the ingredients together and you can get results in half an hour. It's almost as if the diamondoids know where they want to go."
The Smaller the Better
Although there are other ways to get materials to self-assemble, this is the first one shown to make a nanowire with a solid, crystalline core that has good electronic properties, said study co-author Nicholas Melosh, an associate professor at SLAC and Stanford and investigator with SIMES, the Stanford Institute for Materials and Energy Sciences at SLAC.
The needle-like wires have a semiconducting core - a combination of copper and sulfur known as a chalcogenide - surrounded by the attached diamondoids, which form an insulating shell.
Their minuscule size is important, Melosh said, because a material that exists in just one or two dimensions - as atomic-scale dots, wires or sheets - can have very different, extraordinary properties compared to the same material made in bulk. The new method allows researchers to assemble those materials with atom-by-atom precision and control.
The diamondoids they used as assembly tools are tiny, interlocking cages of carbon and hydrogen. Found naturally in petroleum fluids, they are extracted and separated by size and geometry in a SLAC laboratory. Over the past decade, a SIMES research program led by Melosh and SLAC/Stanford Professor Zhi-Xun Shen has found a number of potential uses for the little diamonds, including improving electron microscope images and making tiny electronic gadgets.
Fuzzy white clusters of nanowires on a lab bench, with a penny for scale. Assembled with the help of diamondoids, the microscopic nanowires can be seen with the naked eye because the strong mutual attraction between their diamondoid shells makes them clump together, in this case by the millions. At top right, an image made with a scanning electron microscope shows nanowire clusters magnified 10,000 times. (SEM image by Hao Yan/SIMES; photo by SLAC National Accelerator Laboratory) (click on image to enlarge)
For this study, the research team took advantage of the fact that diamondoids are strongly attracted to each other, through what are known as van der Waals forces. (This attraction is what makes the microscopic diamondoids clump together into sugar-like crystals, which is the only reason you can see them with the naked eye.)
They started with the smallest possible diamondoids - single cages that contain just 10 carbon atoms - and attached a sulfur atom to each. Floating in a solution, each sulfur atom bonded with a single copper ion. This created the basic nanowire building block.
The building blocks then drifted toward each other, drawn by the van der Waals attraction between the diamondoids, and attached to the growing tip of the nanowire.
"Much like LEGO blocks, they only fit together in certain ways that are determined by their size and shape," said Stanford graduate student Fei Hua Li, who played a critical role in synthesizing the tiny wires and figuring out how they grew. "The copper and sulfur atoms of each building block wound up in the middle, forming the conductive core of the wire, and the bulkier diamondoids wound up on the outside, forming the insulating shell."
An illustration shows a hexagonal cluster of seven nanowires assembled by diamondoids. Each wire has an electrically conductive core made of copper and sulfur atoms (brown and yellow spheres) surrounded by an insulating diamondoid shell. The natural attraction between diamondoids drives the assembly process. (Image: H. Yan et al., Nature Materials)
A Versatile Toolkit for Creating Novel Materials
The team has already used diamondoids to make one-dimensional nanowires based on cadmium, zinc, iron and silver, including some that grew long enough to see without a microscope, and they have experimented with carrying out the reactions in different solvents and with other types of rigid, cage-like molecules, such as carboranes.
The cadmium-based wires are similar to materials used in optoelectronics, such as light-emitting diodes (LEDs), and the zinc-based ones are like those used in solar applications and in piezoelectric energy generators, which convert motion into electricity.
"You can imagine weaving those into fabrics to generate energy," Melosh said. "This method gives us a versatile toolkit where we can tinker with a number of ingredients and experimental conditions to create new materials with finely tuned electronic properties and interesting physics."
Theorists led by SIMES Director Thomas Devereaux modeled and predicted the electronic properties of the nanowires, which were examined with X-rays at SLAC's Stanford Synchrotron Radiation Lightsource, a DOE Office of Science User Facility, to determine their structure and other characteristics.