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Uncovering the secrets of MoS2

29 May 2013

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Columbia University researchers have grown high quality MoS2 (molybdenum disulphide) crystals, the world's thinnest semiconductor, which can be either insulating or conducting to form the basic "on-off switch" in digital electronics. They have uncovered some key insights into the new material's optical and electronic properties, as well as how the tiny crystals stitch together at the atomic scale to form sheets.

"Our research is the first to systematically examine what kinds of defects result from these large growths, and to investigate how those defects change its properties," says James Hone, professor of mechanical engineering at Columbia Engineering, who led the study. "Our results will help develop ways to use this new material in atomically thin electronics that will become integral components of a whole new generation of revolutionary products such as flexible solar cells that conform to the body of a car."

This multi-disciplinary collaboration by the Energy Frontier Research Centre at Columbia University with Cornell University's Kavli Institute for Nanoscale Science focused on molybdenum disulphide because of its potential to create anything from highly efficient, flexible solar cells to conformable touch displays. Earlier work from Columbia demonstrated that monolayer MoS2 has an electronic structure distinct from the bulk form, and the researchers are excited about exploring other atomically thin metal dichalcogenides, which should have equally interesting properties. MoS2 is in a class of materials called transition metal dichalcogenides, which can be metals, semiconductors, dielectrics, and even superconductors.

Thinnest semiconductor

A false-color electron microscopy image showing the star-shaped crystals in monolayers of two-dimensional semiconducting molybdenum disulfide. The red, yellow, and blue colors represent two dominant crystal orientations that are stitched together by a line of atomic defects.

2-D crystals

"This material is the newest in a growing family of two-dimensional crystals," says Arend van der Zande, a research fellow at the Columbia Energy Frontier Research Centre and one of the paper's three lead authors. "Graphene, a single sheet of carbon atoms, is the thinnest electrical conductor we know. With the addition of the monolayer molybdenum disulphide and other metal dichalcogenides, we have all the building blocks for modern electronics that must be created in atomically thin form. For example, we can now imagine sandwiching two different monolayer transition metal dichalcogenides between layers of graphene to make solar cells that are only eight atoms thick—20 thousand times smaller than a human hair!"

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," notes 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 three 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 an American football field—and it would not have any misaligned atoms," says Pinshane Huang, a PhD student at Cornell and the paper's third lead author.

Like patches on a quilt

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," says Hone. "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."

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