Creating New Opportunities From Nanoscale Materials

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Moiré Patterns

Graduate student Reidy is interested in how to control the growth of gold on 2-D materials such as graphene, tungsten diselenide, and molybdenum disulfide. When she deposited gold on “dirty” graphene, blobs of gold collected around the impurities. But when Reidy grew gold on graphene that had been heated and cleaned of impurities, she found perfect triangles of gold. Depositing gold on both the top and bottom sides of clean graphene, Reidy saw in the microscope features known as moiré patterns, which are caused when the overlapping crystal structures are out of alignment.

The gold triangles may be useful as photonic and plasmonic structures. “We think this could be important for a lot of applications, and it is always interesting for us to see what happens,” Reidy says. She is planning to extend her clean growth method to form 3-D metal crystals on stacked 2-D materials with various rotation angles and other mixed-layer structures. Reidy is interested in the properties of graphene and hexagonal boron nitride (hBN), as well as two materials that are semiconducting in their 2-D single-layer form, molybdenum disulfide (MoS2) and tungsten diselenide (WSe2). “One aspect that’s very interesting in the 2-D materials community is the contacts between 2-D materials and 3-D metals,” Reidy says. “If they want to make a semiconducting device or a device with graphene, the contact could be ohmic for the graphene case or a Schottky contact for the semiconducting case, and the interface between these materials is really, really important.”

“You can also imagine devices using the graphene just as a spacer layer between two other materials,” Ross adds.

For device makers, Reidy says it is sometimes important to have a 3-D material grow with its atomic arrangement aligned perfectly with the atomic arrangement in the 2-D layer beneath. This is called epitaxial growth. Describing an image of gold grown together with silver on graphene, Reidy explains, “We found that silver doesn’t grow epitaxially, it doesn’t make those perfect single crystals on graphene that we wanted to make, but by first depositing the gold and then depositing silver around it, we can almost force silver to go into an epitaxial shape because it wants to conform to what its gold neighbors are doing.”

Electron microscope images can also show imperfections in a crystal such as rippling or bending, Reidy notes. “One of the great things about electron microscopy is that it is very sensitive to changes in the arrangement of the atoms,” Ross says. “You could have a perfect crystal and it would all look the same shade of gray, but if you have a local change in the structure, even a subtle change, electron microscopy can pick it up. Even if the change is just within the top few layers of atoms without affecting the rest of the material beneath, the image will show distinctive features that allow us to work out what’s going on.”

Reidy also is exploring the possibilities of combining niobium — a metal that is superconducting at low temperatures — with a 2-D topological insulator, bismuth telluride. Topological insulators have fascinating properties whose discovery resulted in the Nobel Prize in Physics in 2016. “If you deposit niobium on top of bismuth telluride, with a very good interface, you can make superconducting junctions. We’ve been looking into niobium deposition, and rather than triangles we see structures that are more dendritic looking,” Reidy says. Dendritic structures look like the frost patterns formed on the inside of windows in winter, or the feathery patterns of some ferns. Changing the temperature and other conditions during the deposition of niobium can change the patterns that the material takes.

All the researchers are eager for new electron microscopes to arrive at MIT.nano to give further insights into the behavior of these materials. “Many things will happen within the next year, things are ramping up already, and I have great people to work with. One new microscope is being installed now in MIT.nano and another will arrive next year. The whole community will see the benefits of improved microscopy characterization capabilities here,” Ross says.

MIT.nano’s Osherov notes that two cryogenic transmission electron microscopes (cryo-TEM) are installed and running. “Our goal is to establish a unique microscopy-centered community. We encourage and hope to facilitate a cross-pollination between the cryo-EM researchers, primarily focused on biological applications and ‘soft’ material, as well as other research communities across campus,” she says. The latest addition of a scanning transmission electron microscope with enhanced analytical capabilities (ultrahigh energy resolution monochromator, 4-D STEM detector, Super-X EDS detector, tomography, and several in situ holders) brought in by John Chipman Associate Professor of Materials Science and Engineering James M. LeBeau, once installed, will substantially enhance the microscopy capabilities of the MIT campus. “We consider Professor Ross to be an immense resource for advising us in how to shape the in situ approach to measurements using the advanced instrumentation that will be shared and available to all the researchers within the MIT community and beyond,” Osherov says.



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