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Thinnest semiconductor grown from MoS2 crystals

Posted: 29 May 2013 ?? ?Print Version ?Bookmark and Share

Keywords:MoS2? thinnest semiconductor? dichalcogenides?

Until last year, the majority of experiments studying MoS2 were done by a process called mechanical exfoliation, which only produces samples just a few micrometres in size. "While these tiny specimens are fine for scientific studies," noted Daniel Chenet, a PhD in Hone's lab and another lead author, "they are much too small for use in any technological application. Figuring out how to grow these materials on a large scale is critical."

To study the material, the researchers refined an existing technique to grow large, symmetric crystals up to 100 microns across, but only 3 atoms thick. "If we could expand one of these crystals to the thickness of a sheet of plastic wrap, it would be large enough to cover a football fieldand it would not have any misaligned atoms," said Pinshane Huang, a PhD student in the David Muller lab at Cornell and the paper's third lead author.

For use in many applications, these crystals need to be joined together into continuous sheets like patches on a quilt. The connections between the crystals, called grain boundaries, can be as important as the crystals themselves in determining the material's performance on a large scale. "The grain boundaries become important in any technology," Hone said. "Say, for example, we want to make a solar cell. Now we need to have meters of this material, not micrometres, and that means that there will be thousands of grain boundaries. We need to understand what they do so we can control them," he noted.

The team used atomic-resolution electron microscopy to examine the grain boundaries of this material, and saw lines of misaligned atoms. Once they knew where to find the grain boundaries, and what they looked like, the team could study the effect of a single grain boundary on the properties of the MoS2. To do this, they built tiny transistors, the most basic component in all of electronics, out of the crystals and saw that the single, defective line of atoms at the grain boundaries could drastically change the key electronic and optical properties of the MoS2.

"We've made a lot of progress in controlling the growth of this new 'wonder' nanomaterial and are now developing techniques to integrate it into many new technologies," Hone adds. "We're only just beginning to scratch the surface of what we can make with these materials and what their properties are. For instance, we can easily remove this material from the growth substrate and transfer it on to any arbitrary surface, which enables us to integrate it into large-scale, flexible electronics and solar cells."

The crystal synthesis, optical measurements, electronic measurements, and theory were all performed by research groups at Columbia Engineering. The growth and electrical measurements were made by the Hone lab in mechanical engineering; the optical measurements were carried out in the Tony Heinz lab in physics. The structural modelling and electronic structure calculations were performed by the David Reichman lab in chemistry. The electron microscopy was performed by atomic imaging experts in the David Muller lab at Cornell University's School of Applied and Engineering Physics, and the Kavli Institute at Cornell for Nanoscale Science.

The study was sponsored by the Columbia Energy Frontier Research Centre, with additional support provided by the National Science Foundation through the Cornell Centre for Materials Research.


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