MDS is distinct from graphene and hBN because it isn't exactly flat. Graphene and hBN are flat, with arrays of hexagons formed by their constituent atoms. But while MDS looks hexagonal when viewed from above, it is actually a stack, with a layer of molybdenum atoms between two layers of sulfur atoms.
Co-author Zheng Liu, a joint research scientist in Lou's and Ajayan's labs, noted the Yakobson group predicted that MDS and carbon atoms would bind. "We're working on it," he said. "We would like to stick graphene and MDS together (with hBN) into what would be a novel, 2-D semiconductor component."
"The question now is how to bring all the 2-D materials together," said co-author Sina Najmaei, a Rice graduate student. "They're very different species and they're being grown in very different environments."
Until recently, growing MDS in a usable form has been difficult. The "Scotch tape" method of pulling layers from a bulk sample has been tried, but the resulting materials were inconsistent, Lou said. Early CVD experiments produced MDS with grains that were too tiny to be of use for their electrical properties.
But in the process, the researchers noticed "islands" of MDS tended to form in the furnace where defects or even pieces of dust appeared on the substrate. "The material is difficult to nucleate, unlike hBN or graphene," Najmaei said. "We started learning that we could control that nucleation by adding artificial edges to the substrate, and now it's growing a lot better between these structures."
"Now we can grow grain sizes as large as 100 microns," Lou said. That's still only about the width of a human hair, but in the nanoscale realm, it's big enough to work with, he said.
Once the Ajayan and Lou teams were able to
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